I'm sure the lifespan of used GPUs is reduced, but it's hard to say by how much. There's probably a lot of variability due to chip yield and operating conditions (especially temperature and ESD safety). But if I could get one at half off, I'd probably take that chance.

I believe miners are more likely to undervolt their cards and with running them at a constant rate, they won’t undergo the same stress from heating up and cooling down each time it gets used.