As others have already said, more than likely used guns are back in the display case for reasons other than being a problem gun.
Once you get to know guns and the gun market, you can walk into a gun shop and pick out the problem used guns without ever removing them from the case. There are certain brands and models that are notorious. I won't mention them by name, I don't want to side track the thread.
On area I would use extreme caution is the 1911 market. The 1911 inspires guys to do work they have no business doing. When they sufficiently screw it up, it goes to the used gun case. Unless you know what you are doing, I'd stay away from a used 1911 for that reason. If you do have some knowledge, there are some great deals to be had.
Glocks are the same way, but they are much easier and cheaper to fix should they have a problem. It is also much easier to spot problems due to the uniformity of the brand.
With all that said, I would not hesitate for a minute to buy a used gun from a quality manufacture and a reputable dealer.