Gunbroker .com is a good resource, however a few cautions should be followed.
1. Always check feedback.
2. If they will not accept returns I would pass.
3. Same for inspection periods, no inspection period especially on anything used and I would pass.
4. Location, I would never send money to anyone without a physical address and without a working phone. In other words no PO Boxes.
Gunbroker .com is like any other online auction house. They cannot vouch for those who sell on their website, they only provide a place for the auctions to be held. I have always asked for a copy of the FFL to be faxed to me prior to making payment, this has always been honored by those I have dealt with and I always offer mine in return.