Do you buy your cameras and lenses second hand? Where?

I've bought used equipment from B&H before. Smooth experience and they used to not charge online sales tax back in the day

My main camera and all the lenses for it were purchased used from mpb.com, and I had no complaints!

I bought my last lens used on the "national" auction site for my country, since Ebay isn't that big for our domestic market. When I buy the next I'll probably get that used as well, it's so expensive with new glass. You can often have it for 60-70% of what it costs new, with like new functionality.

KEF but it stings as a Canadian.
Roberts Camera via eBay.
Never private. Too many scumbags out there trying to unload broken crap. Kijiji and the likes has completely trashed any and all faith I ever had in humanity.

Used Photo Pro, Robert's camera Ebay store, and /r/photomarket is my primary
KEH is my 2nd choice
MBP is my 3rd choice

As everyone else said: yes, but only if I'm going to save 25% or more. Otherwise I'll buy new. Most OE warranties aren't transferrable and I would rather have the coverage if all I'm getting used is a small discount.
IMO, the OG A9 is a steal right now at $1,200 - $1,500 on MPB.
If you're just getting started, or are on a budget, a used DSLR like a D7x00 and some older glass is an excellent starting point.
All the great photos of the past were taken on old gear. There's nothing preventing a good example from continuing to take good photos today.

I bought my Canon EOS 40D via MPB and I liked their service, so no complaints there.

I've used eBay, /r/photomarket, and MPB for buying second hand gear. If you're using any BST platform like photomarket, make sure you use a payment method with buyer protection, and regardless I'd make sure to fully test and inspect the lens upon receiving it.
A friend of mine has bought cameras and lenses off Facebook Marketplace, but sorting through the scams to find a real listing can be really annoying. I've never bothered with it myself.

Ive used keh, ebay, and bhphoto. depends what Im trying to get and where I can find it, for example I got a 400mm old nikon film lens for cheap on bhphoto. but when I was selling my old canon gear the auction was pretty low on ebay so I imagine that would be a great way to get started with full frame digital. I wish I would have kept the camera and had it converted to infrared if I had known it was going to sell for so low, a bit after I sold it my old nikon I got off a friend and had converted to infrared died

If you are in Europe, KameraStore is also quite nice (based in Finland). Bought my first DSLR with an extra lens and a few accessories for a nice price from there. Also, they inspect and test every piece of equipment (secondhand) so you have an added piece of mind.