It would depend on how badly the previous owner abused it. How hard was it OC? Did it run @ 85ºC 24x7 for 6 months?
It might be fine, or it might give you trouble down the road. I would think that if the card was used in mining and worked well at full stress and didn't crash, then it's a good card. The prev owner just did some aggressive burn-in testing on it
As long as it wasn't chronically overheating or stressed too hard and the electronics are all intact and the price is right, go for it.