I don’t get your post, the seller already told you he did not mine on the card, maybe they were honest.
Listen, most Miners overclock GPUs 24hours of the day, most will overclock it to squeeze every bit of MH from the card and only do the thermal pads because the card's VRAM is getting fried, hot! The card begins to throttle so they put the pads to reduce temps for stable MH. The miners end goal is to dump the card on the used market after the deed has been done.
Maybe he was being honest maybe he was lying , i had no way of telling nor would anyone else buying a GPU but it turned up and was working perfectly whether it had been mined on or not ... mining does not kill a gpu or degrade its performance no more than gaming on it does.
When mining ETH which most are currently doing than you will be reducing the core clock and voltage , the only thing that gets overclocked is the memory. when i mine on the 3090 the core is locked at 1110 mhz , undervolted to 0.730mv and the memory gets overclocked to +1250 . temps will sit at 36core/56mem ( was 40/94 previously no active backplate ) ... When gaming everything gets turned up so the core is now +120 , memory + 1250 and voltage as high as it goes , its is actually hotter when i game than when i mine but again no issues. The memory will throttle on 3080/90 when it gets to 110 deg junction temps , 99% of the time this can be sorted with good thermal pads and will bring that down into the 80's.