Craigslist: GPU need help

So me being new to buying oldish GPUs on the cheap, I need help, what do you guys look out for with a good deal when dealing locally? Atleast on amd, nvidia I know, somewhat, but team red on the cheap, I need help, so if some one on the forum can point me on a good direction to go in with getting something, like a 7970 for cheap, like tell tale signs some one is bluffing and when some one doesn't know the price of the card.

you got to be careful with used 7xxx series chips as many of them were mined pretty hard.
ask if the chip was used to mine, ask if it was overclocked or water cooled. ask if they have original packaging and if it was bought used. if possible test the chip to make sure it works and run a benchmark+gpuz to see what happens like it completely crashing or getting really hot and ofc the fps.
also budget? what do you want to use it for? system?