It's the best online auction site. I've rarely had an issue with them over close to 1K transactions. Watch out for shipping and CC charges. Pay attention to seller ratings. If you're a seller, you've got a much bigger audience than just posting local, it's free to list and the selling fees are very reasonable. With that said, if you're you're trying to move a used glock at our above retail (or some other similar situation), it's not going to move. You're competing with the rest of the nation.
I use Gun Broker to sell, when I have an item that has a better market there, than on a local or national forum. I buy often as well. But, as a dealer, transfers are free. And, I can use USPS to ship handguns. Saves me and the buyer quite a bit of money on handguns, and, is the main reason I got my FFL.
I've got a couple unique guns I want to sell. Naturally I want the most I can get and I need to hit a large audience. I could sell them quickly on 24 hourcampfire and possibly here but I'll hold on to them before I give them away. I registered on gunbroker last week but they pissed me off with their "identity security questions".