What should you look for and look out for when purchasing a used weapon? Particularly pistols.
Most used guns aren't used enough to worry about it. I buy used guns all the time without checking anything other than if the gun appears to have been abused in some way externally.
A used gun might have less than 500 rds through it in several years, and I'll put that much through a gun in less than a full day of shooting.
Semi's, take the slide off and check the frame for wear from the slide. Once you know how wear looks after a determined amount of rds, you can pretty much tell if the gun has a lot of rds through it and whether there may be premature wear from slide to frame fit.
Revos, check the timing on the cylinder [ the net is your friend on how to check timing ]. If the cylinder is in time or close enough on each cyliner, the gun is unlikely to be a problem.