Might have a warranty but still means the MTBF has "probably" taken a hammering if used for mining. Of course you're relying on sellers being honest and that's why I'd avoid buying GPU's used unless from a trusted seller.
Also most sellers don't allow for the card having been used for mining when it comes to pricing, they expect their card is worth the same as one that's been lightly used. Extreme example being miners expecting their two year old 1080's or 1070's to be worth the same as a used one bought 2 months ago used for gaming only. People seem to pay the prices though, caring little when it was bought sometimes which surprises me.
Of course, even GPU's for mining could last a long time - it's just the unknown factor. It's expected that something being used 24/7 and being hammered more than being idle, is "probably" not going to last as long as one sat idling or in a machine turned off for 18 hours a day.
IMO most GPU's overpriced when used. Even something a few months old should really be worth 25% to 30% less than new IMO, partly as the warranty is not guaranteed and also it's just worth paying 25% more to get something brand spanking new.
I'd agree about the MTBF but I think you need more information on a cards life to make a good judgment. Unfortunately we don't have gpu stats like total power on hours values might be of more value like they do in hard drives. Then you can compare total power on hours with purchase date. It's a similar to cars and mileage \age
