Ebay has some good deals if your careful. There are a ton of people selling used.
I would be pretty weary of most websites though. I would look at sites like ebay, B&H, Markertek, and Adarama. They have some of the cheapest prices of proper new/used items and won't rip you off.
For your $500 You could look at something like a ZR800 (not the 850) and a Rode VideoMic. That would be a bit under 500, so you could get an extra battery and maybe go to the hardware store to grab some parts for a stabalizer or something like that. And that's buying new. You could save a bit by looking for used.