I have bought and sold several guns on Gunbroker.com and have been 100% satisfied until the last one which was about a month ago. I sold a rifle to a very nice fellow in Florida, and the FFL on his end was a stupid dickhead.
He at first refused to let the guy have his rifle until I mailed him a copy of my FFL. I called the jerk and told him that I did not have an FFL and that it was perfectly legal for a non-licensee to transfer a rifle to a FFL holder.
Then he changed his story when I refused to budge. He then told me that he needed me to fill out a "special form"
that had my fingerprints on it?! I very politely told him that he was full of shit and I hung up on him.
I called the BATFE field office and explained the situation to an agent. The agent was very nice. I gave all of this idiot's contact info and used the phrase "possible firearms theft" in the conversation. The agent called the asshat FFL holder and when I called him back he was sweet as sugar and said that the customer had already picked up his rifle.
This is the only negative experience dealing with firearms that I've ever had and was in no way Gunbroker's fault.