I've sold several firearms over the years on Gunbroker. Every firearm that I've sold, the buyer has always provided a copy of their dealer's FFL and I've shipped to that FFL. I do not have an FFL myself and I do not engage in the business of buying or selling guns.
I just sold a shotgun, and they buyer's FFL who is apparently a home-based FFL said he will not provide a copy of his FFL to anyone who is not an FFL themselves, and that actually the ATF prohibits him from doing so because of fraud or something.
In my own research, I'm not seeing anything saying I must see a copy of that FFL, however, the law clearly states, interstate firearms MUST be shipped to an FFL so...
1) without a copy of his FFL, I really can't be sure I am following the law, and
2) I am required by law to tell the common carrier it is a firearm, and they have usually (always?) asked to see a copy of the FFL I'm shipping to.
The easy answer is of course I just take the gun to someone I know who is an FFL and have them put it in their books and ship it, but I'd kind of like to know if anyone has an answer. Every firearm I have ever shipped the FFL has always sent me a copy.