Mining ETH is very profitable these days, the fees to process transactions are pretty high. What are they mining?
And also why are you going to buy GPUs from miners where these GPUs are already used heavily, it has a lifespan to which I do think you know. Why not buy a new one instead from a computer store instead.
It depends on how old are the GPUs, my first gaming rig was build on a gpu that's been used for mining for a year and it serve me for 5 years before I give it out for free to a cousin of mine, it never died mate, I can definitely buy GPUS from miners if I have no choice, like OP said though, buy em cheap hahaha