SingleSix:
There are a lot of practical things that you can do when shopping on gunbroker.com to avoid getting stuck with a gun that is a lemon.
First off, checkout the feedback for the seller on gunbroker. Only deal with people who have an established record of positive feedback from buyers. Avoid anyone with low feedback scores, any serious complaints, or a lack of any established history.
Require that the seller provide decent photos of the gun. Photos should be bright and clear, and give you a very detailed view of the gun that will show any obvious external defects. If the photos included with the listing are inferior in quality, send the seller a message, and ask that they email better photos to you. Most dealers will have no problem at all in doing that.
Next, only deal with folks who will give you an inspection period, during which you can return the gun for a complete refund, as WildAlaska mentions that he offers. Most better dealers will offer such an inspection period. Throughly inspect and test the gun during that period, to make sure that you find it to be satisfactory.
In fact, you should give the gun a good inspection BEFORE doing any of the paperwork to have the gun transfered to you at your local FFL. So have your FFL hand you the firearm first to inspect, BEFORE you do the paperwork to transfer it. That way, if there are any obvious issues or flaws that an inspection reveals, you can simply refuse to receive the gun and take ownership of it.
You should most definitely do that if you ever buy any brand new firearm through gunbroker.com For if you decide that you don't want to accept it, the gun can go back to the dealer, and can still be sold as new, since no one ever took ownership of it. But if you do transfer it to you, take it home, and then find a problem, then you have put the seller in a difficult bind. For he will then only be able to sell the gun as being used to another person, if you should return it to him.
There are unscrupulous people who do dump problem guns on gunbroker.com However, if you do all of the above, you should be able to protect yourself from being taken advantage of.