A few years ago a co worker got a visit from the ATF at work. Every time his wife would go on a spending spree he had to sell his Glock to help pay some of the bills. This time he sold it to a buddy who sold it to someone else. That someone else used it to rob a few convent stores and got caught.
The ATF told him to cover your hide get a name and address next time. Yes not copping anything down is legal but not checking an ID of someone you don't know could be expensive if the ATF wants to be not so nice that day.
I met a seller once. He turned out to be a dealer of some sort, with forms to fill out.
That wasn't mentioned anywhere in his ads here.