First off, ya gotta figure that all online auction houses have their good and bad policies, and all of them have good and bad patrons. Having said that, GunBroker is very heavily biased towards sellers and the commission that they create. Iif a deal goes bad, expect GB to spank the buyer (often to the point where they'll cancel the buyer's account if the buyer insists on complaining). That's great if you're the seller, but no so great if you're the buyer.
I vastly prefer AuctionArms, and have completed at least thirty transactions there without a hitch. The sellers and buyers there seem to be much more mature and less, well, flighty than what you often find on eBay or GunBroker.