Buying a used gun can be tricky. Most of the time, you do not know the previous owner or what has been done to the gun. Used guns may have been modified in ways that make them illegal or otherwise unreliable. Some key parts of the gun may have excessive wear which can make the weapon less accurate or cause various types of malfunctions.
If possible, always shoot a used gun before purchasing it. Most gun shops will let you fire the weapon if you indicate a genuine interest in the gun. Try to fire at least 2 magazines, and try to shoot quick bursts or rapid fire. What you should look for is any problems with the feeding or ejection mechanisms. This is especially important with handguns and assault rifles which rely heavily on blow back or roller locked design. If you encounter any type of malfunction while firing two magazines it should be a red flag.

Things such as recoil springs and hammers should lock up tight and strike with plenty of force. Recoil springs and firing pins can be replaced and are required to be replaced after a certain number of rounds. However, replacing parts on a used gun is like putting money into a used car. If you are buying it used, you probably don't want to spend more money to make it function perfectly.

Barrel use can be loosely determined by the barrel rifling. Over time, a barrel will show signs of wear by a decrease in visible rifling. This will significantly reduce accuracy at long distance. Some hand guns (like the H&K USP) may show wear on the barrel hood which can provide an indication of how much it may have been used.

You should expect cosmetic imperfections on used guns. Things like holster wear, nicks, and scratches will all happen once a gun is used or handled. You should not be concerned with such things unless you are seeking the gun for a collection. Keep in mind that guns are fired at least once before leaving the factory so they may appear used even if they are advertised as used.