I used Gunbroker.com couple of times. Just buy from Seller/s with high positve feedbacks. Some sellers don't list every detail they're selling so ask first before you bid or buy to avoid furture problems. So far I didn't encounter any problems from Gunbroker.
I have used gunbroker.com at least a hundred times for buying and selling items. Only counting firearms I've bought and sold a couple dozen. Never had a problem. As a buyer just make sure you have an FFL lined up who will accept the firearm being shipped to them before you bid.