Well, there are like three classes of GPU miners:
1) The Expert
2) The intermediate
3) The absolute Noob.
#3 runs their cards at stock, hot, noisy with auto-fan, not optimized for power-saving / efficiency, cooling, or lifespan.
#2 Runs their cards efficiently, using overclock (underclock) numbers they got from YouTube. They may or may not run the fans @ 100%. They may sell their cards when they start to "run hot", because they may not know how to re-paste or replace fans properly. (*I am here.)
#1 Runs their cards fully-optimized for efficiency, runs fans @ 100% for maximum cooling, don't care about the noise, and take cards offline to clean, re-paste, and replace fans, for both maximum performance/watt as well as maximum GPU longevity.
#1 Buying used cards, you have to try to figure out which type of miner they are. Buying cards from an Expert (who probably has multiple rigs, each with multiple cards) is the best, because you know cards have been taken care of and properly refurbished.
#2 can be hit-or-miss to buy cards from. Chances are that the card needs a re-paste, and inquire the condition of the fans. If they were run close to 100% 24/7, then the fans probably need to be replaced sooner than later.
#3, I probably would try to avoid buying used mining cards from. "Run hot and put down wet" is how they ran their cards. Not recommended.