Have him ship it through a dealer that you choose. You talk to the dealer, he ships it to your dealer. Once you check it over you send him the money. If he isn't okay with that, avoid the deal.
You are asking for the receiving FFL to act as escrow agent? Nobody is going to ship anything anywhere without being paid first. If you can find an FFL near the seller that is willing to act as escrow for small fee it'd be fairly safe (for both parties). Beyond that you are playing with fire.