This thread may be in the wrong section but I'll give you my 2 cents.
1. make sure the guy has a good rating and good feedback.
2. Find an FFL that will accept the firearm, ask the seller if it's coming from an FFL or a private person. If it's coming from a private person, make sure your FFL will accept it.
Bid on the gun. If you win then you'll send him the money, Postal money order or a bank certified check, with a copy of your receiving FFL that is signed by the FFL. He will put this in the box for shipping. What ever kind of payment you all agree on, just no cash! That away you can always have proof of the money being sent if there's a problem.
3. This is a very important part, Send the MO and the FFL how ever you want, but make sure you get a DELIVERY CONFORMATION # and INSURANCE. I don't trust the postal office no more then the paper guy. Stuff gets lost, and it's your luck it will be your MO! Spen the extra few dollars to protect your self.
Ask the seller lots of questions and if your not sure ask for more pictures. If he wants to sell it he will work with you. Just make sure he has good feedback Gun Broker is just like E-Bay in alot of ways, just watch yourself, no one else is going to!
My 2 buys off Gunbroker were great, just as all my buys and sales have been on THR, but I still cover my A** as much as possible.