Let's not get in the habit of posting promotional posts every day. I'm not sure what the limit should be but twice in as many days already feels too frequent. I'll leave this up today but I'll remove any more from you that come in the next 7 days. Once or twice per week seems a reasonable frequency as a starting place.
Yeah, IMO the promotional posts really got out of hand on the old subreddit. A certain manufacturer (who do make lovely cases) posted pretty comparable pics every 2 days or so. On the one hand I think we should nurture our vendor community, on the other hand I'd rather see this happening organically with users sharing their experiences with builds that they buy. I agree that a sticky thread would be a good solution, people who are interested could just follow that thread. To give it more freshness, maybe it could be a monthly Vendor news in [August, ...] thread where vendors could post.
I think the only exception should be made for announcements of things like new case files or GitHub repos with Gerbers (which can include a link to the vendor that made it). Only permitting that type of vendor posts outside the vendor thread would encourage vendors to make new source files available.
I would add "and only if it is a fresh build" because while I am interested in seeing new builds, and I do want to support vendors, I don't want to be spammed here. A pinned thread for vendor news as the other commentor suggested would do just as well for info.