Also check out Armslist.com
I have bought and sold quite a few guns on that site and besides a couple of creepers and d-bags trying to lowball my price, it has worked really good.
Check out gunbroker.com
Pretty cool site and I've used it a ton.
It's easy to register, buy and sell. Just make sure that you have a FFL dealer to receive and register your guns to you that you buy online.
Zeke