I know most of the less expensive used hardware is going to be server-shaped/rackmount. Don’t go for it unless you have a garage or shed that you can stuff them in. They put out jet-engine levels of noise and require god tier soundproofing in order to quiet them. The ones that are advertised as quiet are quiet as compared to other server hardware.
You can grab an epyc motherboard that is ATX and will do all you want, and can then move it to a rackmount later if you end up going that way.
The NVIDIA launch has been a bit of a paper one. I don’t expect the prices of anything else to adjust down, rather the 5090 may just end up adjusting itself up. This may change over time, but the next couple of months aren’t likely to have major deals worth holding out for.
Don’t worry about how a video card was used. Unless it was handled by howtobasic, they’re gonna break long after they’re obsolete. You might worry about a bad firmware setup, but you avoid that by looking at the seller rating, not the video card.
there’s an argument to be made that a mining gpu is actually the better card to buy since they never went hot>cold>hot>cold (thus stressing the solder joints) like a regular user would do. But it’s just that; an argument. I have yet to find a well researched article on the effects of long-term gaming as compared to long term mining, but I can tell you that the breaking point for either is long after you would have kept the card in use, even second or third hand.