Gunboker, gunaction, and gunsamerica. Yes, you need to register with each, since you would be bidding on merchandise there.
When bidding, and considering how much you want to bid, remember to calculate how much you will pay for shipping and for dealer's fees to recieve it. If you do not have a local gun shop that will receive it for you, both gunbroker and gunauction (formerly auction arms) have a "find a ffl" feature on their homes pages.
What we have here is... failure- to communicate.