Old Joe--
are you a law school professor in another life? That question is a lovely set up that would give the little buggers (your students) a chance to show off.
But since you've called the first FFL the 'seller' and the second one the 'buyer', the answer to what you've asked is a question of federal law and I assume BATF has a regulation covering disappearance in transit. OTOH, everybody else has a piece of the answer under the applicable UCC, the insurance coverage issue and so on. Of course, under the UCC the parties can negotiate terms (FOB, COD, L/C, you pick), but in a private party transaction that doesn't happen.
Laissez les bon temps roulez.
