The law doesn't require any proof.

I figure I have either emails via armslist in my folders, cell phone calls, or private messages on a forum, so I have a trail via those devices that can be traced/tracked. Maybe I am wrong, but I am not doing anything illegal in selling a gun to someone that appears to be able to purchase one, so I am not going to sweat it. Conversely, I assume if I sent an email or a PM on purchasing a gun, there is an electronic trail of me purchasing one. I've had one person have me fill out that form, he never looked at my driver's license, I could have put anything down on that form and he wouldn't have known any different.

The only question I have is, do I need a tin foil hat for thinking there are all these electronic records, or do you need one for thinking you need the form?