I bought a used Glock 17 and found that I probably paid a little more for it (compared to a comparable used handgun) just because it is a Glock. The name seems to command a slight premium. But I saved perhaps ~ 25% over a brand new one.
What to look out for? Same as in every used gun purchase: how was it used, how old, ex-Police, etc? Check out the wear patterns, disassemble and look at the bore/throat and feed ramp, check for tightness and work all the functions. Check wear patterns from both storage or carry. How many mags do you get? Ask for a timeframe to return it if it doesn't shoot or feed properly or fails to function such as a slam-fire.