Difficult to know whether mining is any worse on a GPU than constant gaming.
When you mine you generally run the GPU at about 60% power state (or even less) and at a low temperature with very little cold/warm cycles. Arguably a gpu that has only been mined on likely hasnt had as much wear as one that has been used by a keen gamer. The only thing might be the fans though, which will have been running for a very long time. Also, i believe the memory on 3080's can be prone to overheating when mining so that might be something to be aware of.
Also, i don't believe there is anyway for a manufacturer to tell if a GPU has been mined on (to void the warranty)
Mining wears out the VRAM not the gpu. This is why GPU makers state in their warranty if you mine on it the warranty is void, but again they can't prove you mined on the card unless you tell them.
I personally wouldn't touch a mined on card no matter what anyone is telling me about the conditions it was kept in. Also this generations cards are expensive and I wouldn't want to end up with an expensive paper weight that the miner for sure will not be helping with warranty after if there is even a warranty still on the card as most miners are stripping the cards down and adding new thermal pads to reduce VRAM temps and new thermal paste, meaning they have broken the warranty void stickers on most cards. The FE cards don't have the stickers but we have seen the people modding their 3090s and 3080s with new thermal pads.
This is my opinion and my cards are used for work and the odd game. So it is important for me the GPUs are reliable and not been hammered 24/7 and causing the VRAM to degrade.. Why do you think so many miners want to lower the temps on their VRAM so much even when they were told 105c is fine under normal use, because mining is not normal use on the VRAM and the none stop read and writes does wear it out. Anyways there will be many here now screaming I know nothing and Linus proved an old card mined on and a new one was fine. ... A card with very different VRAM and a card that ran at very slow transfer/write rates. Also Linus is a miner and uses the systems he has in the office and mines on them when idle.
Check his new video where Antony is fixing his pc while he goes off on holiday. The system was mining and even Antony states that in video below and other videos I have seen him Linus state the editing rigs when idle are mining, so a miner is not going to say hey yes mining wears out cards, also he gets most of his cards for free as we know, so why would he care. Miners here in the forums are going to now come hammer this thread with why I'm wrong.. But not gonna reply past this post as this is the graphics section. To answer how much price reduction per year on a used mined card well ... how long is a piece of string ? To me a mined on card is worthless and would never buy one, unless they prove never opened up and comes with warranty remaining that I am happy with, if not ZERO is its value to me... Reality is the miners will try get back what they paid for them or what the market value is at the time, so if cards went up in price they will ask for more than what they paid for them..ROI..