$600 actually seems kind of low for what you describe. Perhaps a peek at GunBroker.com for something similar would give you a better idea of what the "going price" would be, or at least what the paying market is willing to spend on it. If you have actual access to the handgun, a quick disassembly and inspection may reveal something not volunteered in it's description. More information or history on it may be helpful as well. Owner didn't like it and just wants to unload it, got it trade for something else and doesn't like .40 or whatever it may be.
Caveat Emptor. Just sayin'.