Have a look at the pictures of the pistol in my "Surplus Berettas at Lever Arms" thread. That's basically what you are going to find in the $500 range. Functional, accurate, but not "nearly new" looking or anything.
From watching the used cabinet at Reliable gun,it seems to me that inexpensive used 9mm pistols sell much faster than the bigger calibres, probably because the ammo is cheaper. Keep your eyes peeled in the EE for good deals.
A Glock in the $500 range is pretty much going to be a police surplus gun as well. These pop up pretty regularly. The police surplus Glocks I have seen lately have been Glock 22's (.40 cal) though. Any decent condition Glock 17 for $500 won't last long. You are not going to see a 19, 26 or 34 for $500.