The seller does not have to be an FFL or send through an FFL, he only has to send to your FFL.
But, as hotpig mentioned, there are a few FFLs who won't receive a gun from anyone but another FFL.
Most, though, just require that the seller include a photocopy of his driver's license so that they can log it into their books. A few will also want the seller to include a signed statement saying that it is his gun he is sending to Dealer X for transfer.
Make sure the seller does include a copy of your name and address so that the dealer will know who this goes to when it arrives.