To echo the other commenters, mining is pretty gentle on GPUs insofar as constant use goes. ETH is memory bandwidth limited, so cards can be heavily undervolted and underclocked (while overclocking memory) saving power without negatively affecting hashrate. More power is wasteful. Typical ETH mining GPU temps are around 60C. I'd feel ok about buying a used card off someone who knows what they're doing mining.

The 3080 and 3090 are the singular exception. The GDDR6X runs extremely hot and frankly I think it's going to cause a bunch of early failures for even non-mining cards.