One reason that pawn brokers can sell new guns cheap is because they already have a FFL to deal used guns. They can use their FFL to order new guns from wholesalers, so many of them will charge you 10% over their price to order you a gun! Thats not too bad of a profit for ordering you a gun, while they are sitting there minding the pawn shop anyway!
Also, you can get some pretty good guns used at pawn shops. You just have to know how to look for quality. Most pawn dealers do mark their used guns pretty high, but most of them will also deal with you too!