I would also get a photocopy of the other guys driver's liense if possible and include a small statement to the effect that " I john(purchaser) doe am a citizen of the Unites States and have never been convicted of afelony or act of domestic violence andI amnot legally barredfrom owning or purchasing a firearm."
Don't know the specifics but i would think that if you sell to acriminal who is simpy trying to void a nics check there may be a problem and something like this may at least put you in the clear and leave the blame on the criminal where it belongs. Check with a lawyer or your local police for details.