Gunbroker is just like ebay, a place to buy and sell. Sellers make their own terms and conditions as long as they conform to gunbroker's rules. I can't imagine a seller wouldn't ship to your FFL once you make your payment and the FFL forwards a copy of his/her license to the seller. Does the seller have good feedback?
I bought 2 pistols from sellers on gunbroker and had excellent results.
Send the seller an email to clarify, I bet you'll be ok. Good luck.