The gun must be sent to an FFL in the buyer's state. The FFL will handle the transfer.
Unless YOU are also an FFL, you do not need (and probably won't get) a copy of the other guy's FFL. But, he should be able to supply you with his FFL number, which you can verify on the EZCheck site.
204.254.113.252Legally, a non-licensee must only "reasonably believe" that he's sending the gun to an FFL. Only another FFL needs a copy of the receiver's FFL with a "live" signature. Due to fraud-avoidance, most FFLs don't like to give out copies of their license to non-FFLs.
-Troy