As the folks above say, the seller HAS to have a copy of the recieving FFL holder's FFL in hand before they can ship the gun. Some want the copy to indicate that it is for you. When I asked CDNN about it, that is what they stipulated.
Gunbroker has a tool to help find an FFL holder that will do the recieve. Not all will, even though it is free money. A local dealer wouldn't as he wanted to sell you the gun, he is now out of business