As long as you, the buyer, receive the gun from an FFL after doing the proper paperwork it is perfectly legal. The terms "sale" and "purchase" are totally irrelevant in the transaction. The concern comes with the person receiving the gun being the same one that completes the form and passing the background check. Handing guns out for little or no payment is one of the things Clinton and Brady tried to attack but couldn't get done, so it is legal (at least in my experience).