Im building a gaming pc because Ive just got Rome 2. Do you think its a good idea to get a 7950 previously used for mining, or are they just gonna die on me?
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.