The components on a graphics card (VRM, cooler, even the GPU and memory to some extent) ultimately have limited lifespans. How long of a life depends on the conditions in which they"re kept, but run a graphics card long enough and it WILL fail.
Mining cards are typically run 24/7 for extended periods of time, often not in the best conditions. Consumer cards aren"t designed for that.
When you buy a GPU that"s previously been used for mining, you run the very real risk of getting a card that"s on its last legs. While you might get a card that has spent minimal time on the burner and can potentially run for years to come, you might also get a card that has spent the last year sandwiched between other cards and hitting up against thermal limits.