IANAL. I'm going against the grain and say anything illegal the buyer would be doing is between them and the FFL. There's no "paperwork", only
a shipping label being created between the seller and the FFL. Whomever picks up the firearm and fills out the paperwork and background checks
would be answering the questions.
I could see situations where someone buys a firearm because their friend/family doesn't have a gunbroker account, and has it shipped to an FFL
in the name of the friend/family. I don't believe any laws would be broken?
But it's your ultimate right to walk away from any buyer you feel doesn't give a legit good feeling.
again, IANAL
SO, if you suspect that someone is buying a firearm from you for a likely prohibited person you feel no responsibility at all to shut it down? Let the FFL deal with it?