How do you tell there's no corrosion though? You need to at the very least take off any coolers on the card and look at the PCB itself to have some level of con

How do you tell there's no corrosion though? You need to at the very least take off any coolers on the card and look at the PCB itself to have some level of confidence that there's no damage, and even then I've seen corrosion sneak under components where you need to pay really close attention to notice it. If I can inspect a card in person, I would be plenty happy to buy an ex-mining card, but most of these are going to be sold online sight-unseen. This isn't a problem exclusive to mining cards though; people have been selling damaged GPUs as 'working' for years.