Audiogon is a great option. With used there's some risk, but if you stick to sellers with good feedback you're usually safe. I've bought and sold quite a bit of stuff there with no problems (I just got a Margules Daleth Multichannle Preamp for about 1/2 what a new one cost, fwiw).
You could get a very nice 2CH stereo pair for under $500 used. Even new that will buy nice ones. 5.1 is a bit harder- used you could probably get the main 5, but not much of a sub. That's not really a dealbreaker, though. A surround rig can sound great without a sub, and when you add one later on it's just that much more improvement.