I have never purchased anything on Gunbroker, but I do browse it often. I use it for an idea of the going price on things (not the asking price, but look at actual bids and closed auctions for what things sold for). I have made a handful of purchases on e-bay, back when they allowed reloading stuff.
The key to Gunbroker is as others have said to look at the ratings on the seller, do the homework on what you are bidding on, and Know What You Are Willing To Spend!!!
I'm horrible at live auctions in that I can get caught up in the bidding if I'm not careful ... Really sucks when your bidding on trucks and equipment in the tens of thousands. Bidding on-line is just as dangerous. Don't forget that if you go $10 too high on that 10/22, it will get even worse when you add shipping and the FFL.
Outside of all these warnings, I've never had a bad contact experience with gunbroker, and I do know a couple people that have purchased and been pleased.