Gunbroker is awesome, if you're patient and watch the non featured items you can get some crazy deals. Also poke around for stuff that was spelled wrong.
Edit: And love the 15 min rule, eliminates all the sniping you get on ebay type sites where people bid at last second.
Yes, it seems you can submit a maximum price and still pay below that price for the item if other bidders don't drive up the bids.
So what happens if someone bids a higher maximum price than you?