Does mining shorten the life of a GPU?
If I am in the market for a second hand GPU for gaming, should I avoid those?
Generally, yes, as miners tend to tweak GPU settings to maximise their hashrate and it can cause issues when they are run like that long term.
That's much better for the GPU than, say, pushing the card to the limit for benchmark scores :D
But generally, GPUs become obsolete before they die.