Why is it legal to buy and sell used books/discs but illegal to buy and sell used digital files?

In theory, sharing a digital file can have a much greater impact than sharing a CD physically. Plus, you lose access to your copy of the CD if you give it to someone else. You can think of it like transferring a license for one user to a different user. There is no simultaneous usage.
I don't personally agree with this view, but I believe that's the argument.
- I buy disc.
  I rip contents of CD to computer.
  I sell disc.
  
  DON'T COPY THAT FLOPPY!!
  This argument is only a "gotcha" if it was permissible use, but it wasn't, even before CDs.
  
  The amount of people who will duplicate their tapes and CDs would be lower than the amount of people who will duplicate their digital files.
  Most of the time when a law sounds silly for banning something when alternatives exist, it's because people themselves are silly and don't actually go for the alternatives at the same rate as they would the banned thing. Ie gun accessory bans, ninja star bans.
  
  Netflix's mail service was great for data hoarders.
  
  Of those three steps, step 2 is the illegal one. (Assuming we're talking about music and not software.) Even if you never do step 3.
  (Not saying things should be that way. Nor that it's not difficult to enforce. Only that as the laws are today, even ripping a music CD to your hard drive without any intention to share the audio files or resell the CD, even if you never listen to the tracks from your computer, the act of making that "copy" infringes copyright.)
  Edit: Oh, and I should mention this is the case for U.S. copyright. No idea about any other countries.
- which is eyactly why piracy isn't theft.
  it can still be a crime, just don't call it what it obviously isn't.
  
  It isn't piracy, either. It's filesharing.
  See Richard Stallman, "Ending the War on Sharing":
  When record companies make a fuss about the danger of "piracy", they're not talking about violent attacks on shipping. What they complain about is the sharing of copies of music, an activity in which millions of people participate in a spirit of cooperation. The term "piracy" is used by record companies to demonize sharing and cooperation by equating them to kidnaping, murder and theft.
  
  It never used to be a crime. Bastard lobbyists!
- There are companies out there that do allow this for digital licenses. Arturia, an audio software and hardware company, lets you de-register and sell a license key to someone else, who can re-register it. They don’t charge any fees for it at all either, like some companies do. It’s not hard, most companies just don’t care about you as a customer.
  Edit: Their license keys all include five seats too.
- And that's why you don't own digital media but only a "usage license", because the original owner still has the original? Isn't it then fraudulent if the shops sell you the media, despite it being only a license? And shouldn't that be cheaper then?
  Well, anyway, harr harr.
- Can't you transfer a game to other people on Steam? They treat it like a physical item where after giving it away you loose access IIRC.
  
  Not if you already activated it.
  It used to be possible to buy games as gifts and and them to your inventory to give to somebody ( or activate it yourself ) later.
  Now, when you want to gift a game. You have to immediately select the person you want to send it to.

There's a few things about this
Many times you don't own the digital good, you subscribe to it. No I'm not joking, that's why services can usually take it away at any time. You normally own "a licence to play it on a single PC" or similar.
This isnt apples to oranges per se. Selling digital goods is fine, it's copying it. Similar to how photocopying a book and selling it would not be okay.
It's important to note there is a narrative push by companies too. They spend lots of money putting videos on every DVD saying "downloading is stealing" because if society thinks piracy and stealing is the same, it helps them litigate and make more money.
Your idea of a lost sale is a hard one, from a media company point of view, it's about making money. So if you can make people believe "a download is a lost sale" or "sharing a digital file is a lot sale" etc, then you can use that to sue individuals, isps, sharing sites, search engines etc and make more and more money while also having more power over your product.
- if you live in the EU you own your digital purchases.
  
  It's kind of not tested though.
  If you've never been given the option to download it and save it and use it from there, how would you "own" it if the streaming service takes it offline?
  If you can't transfer ownership of something, or have it past the lifespan of the shop you bought it from, do you really own it? I would say not.
  
  You do not. Or we have a different definition for owning.
  
  I'm in the EU. How do I download movies I purchased on Amazon?

There is no difference, they just want you to believe there is a difference.
Cos if you believe it then they can make more money.
The point: GREED that's the difference.
- Dude, of course there is a difference. If you sell a physical good you do not have that physical good anymore. If you sell a digital copy you can keep selling that digital copy because you do not necessarily give it away or delete it.
  Saying that there is no difference at all between the two is silly.
  To be clear, I am not saying this justifies anything regarding copyright, but it is a difference if you can sell something over and over again or just once.
  
  If you sell a digital copy you can keep selling that digital copy because you do not necessarily give it away or delete it.
  Steam and other DRM systems ensure that copies cannot be played. Yet you can't sell your Steam games. It is my understanding that in the EU, you can sell your Steam games. So there is no legitimate reason you can't sell digital goods.
  
  Thats why copying of an immaterial gpod is not stealing because the other person still has the full ownership over their copy.
  
  There were lots of book copying back in the day, and disc copying too ofc.

Because the book and disc guys couldn't figure out a way to stop you back then.
Nowadays college books have one time codes for tests, and games will sometimes have codes included for inportant unlocks to force used purchasers to pay up.
- I've not seen that in games for a while tbh. I think "Project Ten Dollar" died a death and was replaced with a constant barrage of micro-transactions and not so micro-transactions, sprinkled with FOMO dust.

One thing to keep in mind that may be relevant: copies of non-digital things are different than digital copies.
Digital (meant here as bit-for-bit) copies are effectively impossible with analog media. If I copy a book (the whole book, its layout, etc., and not just the linguistic content), it will ultimately look like a copy, and each successive copy from that copy will look worse. This is of course true with forms of tape media and a lot of others. But it isn't true of digital media, where I could share a bit-for-bit copy of data that is absolutely identical to the original.
If it sounds like an infinite money glitch on the digital side, that's because it is. The only catch is that people have to own equipment to interpret the bits. Realistically, any form of digital media is just a record of how to set the bits on their own hardware.
Crucially: if people could resell those perfect digital copies, then there would be no market for the company which created it originally. It all comes down to the fact that companies no longer have to worry about generational differences between copies, and as a result, they're already using this "infinite money glitch" and just paying for distribution. That market goes away if people can resell digital copies, because they can also just make new copies on their own.
- it will ultimately look like a copy, and each successive copy from that copy will look worse. This is of course true with forms of tape media and a lot of others. But it isn't true of digital media, where I could share a bit-for-bit copy of data that is absolutely identical to the original.
  There is one exception: reposted memes, they are losing pixels more and more. /s
  
  Only because of incompetent people using lossy reproduction methods out of ignorance
- What about physical media containing digital data, e.g. a CD?
  
  It's technically illegal to make a copy of that data for yourself and then to sell the original (while keeping the copy). That obviously doesn't mean it doesn't happen, but...
- You cannot endlessly duplicate digital data without some loss.
  
  This assumes a conversion on each copy. That's not how digital copying works. We CAN share the same file indefinitely by copying the data without loss indefinitely. It's when you transcode/reencode the data that you introduce loss.

- Ahem actually it's because 🤑 💰 💸
  
  ...of the greedy industry, not the artists

Physical media doesn't have the luxury of endlessly replicating itself via a simple copy and paste.
- Distributing data to everybody is the only communism that works.
  Endless replication, creates endless intelectual and creative wealth and diminishes financial wealth to zero.
  
  It won't even diminish it to zero. Some people will still pay. Ya I could download games for free and play them but I want to support the teams that made them so they keep making them. I only get games for free if that is the only way of getting them.
- If a copy isn't kept after sale, it should be legal. It's my understanding it is legal in the EU.
  That is if you can sell a game you bought on Steam. Steam makes sure to copies aren't played at the same time so you can't keep your copy after selling it.
  
  From my understanding, you don't "own" a game you bought on Steam, you just own the license to play it. The game file without modifications is protected by DRM, and only works when it's launched from Steam with a valid license. Notice when using the same account on two different PCs, Steam would force quit the game when you try to launch the same game from the other PC.
  In a closed system like Steam, sure, it would be relatively easy to regulate the buying and selling of game licenses since you're doing it all under Steam's system. When Steam detects a license transfer or however they want to implement it, they can easily disable access for the seller and enable it for the buyer.
  But if the game file is DRM free, then it's the same as downloading pirated movies, there would be no guarantee that the seller has no access to the game after selling it. No way to regulate it either. Hence, endless copies.
- Since when, you can do a perfect rip of a CD and burn an exact image of the disk loslessly thousands of times. Same for DVD and Blu-rays. If you are talking about a physical book, then yes, making lossless copies is more involved, but still technically possible with the proper equipment and knowledge.

If they could, they would. Laws aren't passed because it makes sense, but because they benefit the rich.
- "The law, in its majestic equality, forbids the rich as well as the poor to sleep under bridges, beg in the streets and steal loaves of bread."
  -Anatole France

Logically yes, downloading or a sharing a digital file is a "lost sale" - but as Aaron Swartz said - "lost sales" are also caused by Earthquakes, Libraries/DVD Rentals, Negative Yelp reviews, Market Competitors, and so on. Why just blame it on file sharing...

It's in the name. Copyright. As in, the right to make a copy.
It's perfectly legal to sell a digital good as long as you don't retain it as well.
It's illegal to make a copy of a book and then sell that copy.
From probably the most biased source possible: https://copyrightalliance.org/education/copyright-law-explained/limitations-on-a-copyright-owners-rights/first-sale-exceptions-copyright/
As they point out, most digital works are licensed, not sold, so there are terms and conditions associated with how you can use them.
So it's perfectly consistent, just grossly out of date for it's intended purpose of "make sure writers can make money selling their books without worrying that getting copies made will be pointless because someone else will undercut them and leave them with 1000 prepaid copies of their book that everyone bought cheaper".
We should have a system that preserves that original intent of "creators get compensated", without it turning into our culture gets owned by some random company for more than a lifetime.

Legally speaking you don't own the digital files, you have a license but you don't actually own it.
- So, why can’t I buy and sell license?
  
  Ehhh they’re technically wrong. You don’t own the media with physical formats either. There’s just nothing they can do about it if they want it out of your home.
  
  In a way, NFTs were supposed to solve this.
  
  I believe there was a recent EU ruling that mandated that this must be allowed.
  I'm not sure of the details, however.
  
  You literally can. Check out kinguin
  
  Often, licence agreements stipulate that they are not transferable and thus you have contractually agreed not to resell them. To what extent this is enforceable is... contentious. Different courts have struggled with the topic and have ruled both directions on the issue.
  Copyright law as written was not designed for immaterial goods in any way, and the DMCA has done little to improve that. So effectively the judicial branch is in limbo. Corporate America is content to leave the confusion as is. They can just adopt an interpretation of the law that is maximally beneficial to them, and consumers generally don't have the resources to challenge that interpretation.
- That is no different with CDs, DVDs, etc. It’s written on all of them. It’s a limited license, which is why you can’t host screenings and charge people with your physical media.

Its legal in the EU. You can sell your windows key for example, despite the windows eula disallowing it

Because companies have money, and money makes laws.
- because making a legitimate copy of a book is a lot more work

- Iirc there was an online ebook market in the Netherlands where you could resell your ebooks. This was deemed illegal because there is no way to guarantee the seller has deleted his own copy after it was sold.
  
  Crazy.
  It's not too much of a stretch to apply that to selling a CD; the vendor would have to prove that they didn't make a copy?
  Guilty before proven innocent.

Both require a license and that license is revocable in both cases. It's pretty much impossible to enforce the legal use of physical media, so they don't. Rather, they go after those making copies.
If you ever want to REALLY own your media, make sure it is physical.

It's not? I see digital files being bought and sold all over the place. 3rd party key retailers and even the digital goods earned within games themselves. It's only illegal for you to copy and sell copies of digital goods. If you have a way to sell the original thing and no longer have access to it, it's perfectly legal. It's just not many things let you do that.

Usually, when the question is "why is copyright restrictive in these evil and/or dumb ways" the safe bet is that the the answer is "Disney."

Because they somehow don't consider that someone could have copied the books or discs before reselling them, but it immediately comes to their mind with digital copies riddled with DRMs

Because lawyers, accountants and MBAs making policy for things they don't understand.

The legalese in the US (which might as well be everywhere as you need to have compatible copyright with the US to have a trade deal with the US, and your country is in trouble if it doesn't have a trade deal with the US) is basically that:
If you buy a physical copy, you've become the owner of that one copy of the IP contained within. As the owner of that copy, you can do stuff with it like read it, display it, destroy it, or sell it on to someone else thanks to the First Sale Doctrine (but you can't do other things like copying it, unless it's a DVD as there's a specific exemption for the copy your DVD has to make to RAM in order to decode the DVD). There's nothing the copyright holder of the original can do to stop you exercising these rights.
If you buy a digital 'copy', you've not bought a copy, you've bought a licence to use one of the publisher's copies that they've given you permission to have on your device(s). They'll also have given you permission to do things like read it if it's an ebook or play it if it's a video game, but as it's their copy, not yours, you don't automatically get rights to do anything they've not explicitly permitted you to, and it's not in their interests to permit you to sell it on unless they think you'll pay enough more for a resaleable copy that it covers a potential future lost sale.
I'm sure plenty of publishers would love for the second set of rules to apply to things like books, and from a quick googling, it seems like occasionally academic textbooks have included a licence agreement instead of you actually owning the physical book, but I imagine that most publishers are concerned about bad PR from attempting this with a hit novel and also don't want to be accused of fraud for having their not-a-book-just-a-licence on the shelf next to regular books and thereby tricking consumers into thinking they were buying a regular book. EA attempted to double-dip over a decade ago with Battlefield 3, which included a copy of the game (with regular First Sale Doctrine rights) and a licence key for the online pass (which wasn't transferrable) and got bad press because of it. Newer PC games often come as a key in a box with no disk or a disk that only runs a web installer, so you've not got a copy of the game to claim you've bought and obviously only have a licence, and this seems to have caused less upset. This wouldn't work with a book, though, as you have to fill in the pages at the printing factory, and can't magically do it only after the user's got it home.
- Just a small point on the first comment (and I wish I could find the article to refer to, but the internet is a giant void that buries things faster than a mudslide). There was a case where a game developer sued, and I believe won, against a homeowner who was selling one of their games at a garage sale because, even though they had the physical media, the EULA explicitly revoked transferral of possession and was, in fact, just a license, not ownership. I remember it vividly because it was the first time I had ever heard of someone claiming that someone did not own a physical object that they purchased in a store. Afterward, I discussed it with some of the used game retailers and found out that there were some games that they were specifically prohibited from accepting as trades for this very reason.

bourgeoisie lobby in the State

“Reasons”

Pretty sure they tried to prevent the sharing and reselling of CDs at one point.
- Look up the history of DIVX DVDs. Not to be confused with the DivX video codec.

The theory probably revolves around there not being a transfer of the file as much as a copy paste of it.
Physical media can be copied but you need blank physical media to copy it to, in a purely digital eco system both the buyer and seller end up being able to have copies of the sold product, which effectively treats the seller as if they are a vendor of the product being sold.
Basically in a world built on copyright law, being able to buy and sell digital media the same as physical media looks a lot like someone scalping the original product to cut into the maker's bottom line. Megacorps eating it is pretty dope but it significantly diminishes an individual developer's ability to profit off their own work as well, especially when software development already encourages so much copy pasting to make software that should be working into software that is working.

Is it actually illegal? Has anyone in the US ever been charged for selling digital software they had legally acquired?

Because you rent them and not own them. It's also illegal to sell a book that you rented from the library. Or get a dvd from the library and then copy it. It's a measure they put into place so you're not allowed to duplicate the thing. Hence they don't grant you the same kind of ownership you'd have over a physical item.

I believe selling and sharing ebooks is illegal too. So digital and physical contents is different

Under US law (I see someone else posted about EU law):
Physical property has a long tradition of legal rights that are a part of ownership. There's a thing called "right of first sale" that means you have the right to sell an object that you own. This legal framework falls under property law, even if the media on the disc is also governed by copyright law. In this case, property law is inviolate - it trumps copyright law.
Digital files are instead governed only by copyright law. Further, media companies could not modify copyright law fast enough to keep up with the digital revolution, so more than copyright, digital files are controlled by digital rights management (DRM) code, and contract law (the long TOS when you access a service or site).
The contract law in the TOS, and code in the DRM, do two things: they force a digital file owner to treat it as a "license," and give media company the ability to severely restrict use after the purchase.
So basically, when you buy a disc, you are simply getting a lot more rights to use that content. You literally own the copy.
This is why media companies are doing everything that they can to switch customers over to streaming services and stop selling physical content. It's also why it's a literal lie when you are told you can "buy" digital copies that have DRM, because those companies will simultaneously charge you the higher "purchase" price and deny you ownership rights as if you bought a disc.
- Point of order, gaming companies have been pulling this shit since before digital sales. The first time I encountered it was a developer suing a homeowner for selling a game at a garage sale that had the EULA/TOS clauses that it was a license, not an ownership. I was dumbfounded because I had always believed that personal property law trumped copyright, but this was not the case. I have actually heard some old stories about book publishers trying to pull the same shit, but I think ownership did win in those cases.
- Maybe things have changed, but in the US it used to be a question of when does a digital file "exist". The law was: when you buy a digital asset, the first time that asset is instantiated is the one true version of the file. To sell it you MUST sell the device where it first appeared. So you have to sell the computer, phone, or tablet that was used to download it. Maybe that law has been revised.

This may be a dumb question, but why can't crypto something or an NFT be imprinted on my copy of the album/picture/whatever so I could sell it and lose my access? It's this a function of no standardized marketplace for digital goods and services?
- Because crypto is a joke and NFTs are vaporware. The concept is honestly laughable. Perfect example for NFTs, my company wanted to advertise that our service could help with the production of NFTs and my boss had put together the ad for it. I advised against it considering the BS and controversy associated with it. It became doubly apparent when I looked at the ad and saw that he had included several of the (in)famous NFTs that had been sold. I point blankly asked him if he had gotten permission to use them. He said no. Then I pointed out that the fact he was able to put them in the ad without asking and paying for the right to to the person who had spent millions on the NFT was exactly my point. NFTs are a scam. Thankfully he saw the light and dropped the whole nonsense.
  As for the blockchain in general, it is unsustainable. It requires enormous amounts of power and computing cycles to maintain which gives it a massive eco-footprint and sucks resources needed by actual industries and individuals to support. If you started attributing it to all digital purchases, including resales, it would expand exponentially. It is fine in concept, and if it could function in a passive state somehow it might see usefulness as a purchase and resale history for digital media, but it can't. It requires many computers maintaining identical records in active communication with each other.
  
  Very enlightening. Thanks! So there's still some big hurdle of what would be standardized to make resale of digital goods make sense otherwise it's impossible to police who does and doesn't have legit copies or who made a copy of the file, etc.

Y'all don't really give a fuck about the actual musicians, do you.
- Jesse what the fuck are you talking about
- In contrary. I do. But I fucking hate the whole useless, greedy industry that sits on the artists neck. I prefer p2p transactions so that the artists I like can keep producing more of their art.