FYI, I'm the seller, and I have done sales this way in the past for a couple of reasons:
#1: To make sure the guy I'm dealing with is of age, and the actual person he says he is.
#2 If I am contacted by law enforcement later on, and the weapon was used in a felony
after I sold it, then I can prove it was not in my possession.
#3 If I am the buyer, and the seller just sold me a hot gun, or one that has been used in a crime, and somebody shows up at my door aking me to prove when I purchased it, and explain how I came by it, I have something more then "I bought it from some guy, and I don't remember who" to cover my own tail.
I've e-mailed the buyer, and I can see why he may be uncomfortable about giving out a copy of his DL, so if we meet, I'll just let him prove he's of legal age, not make any copies of his DL#, and at least have a name on the bill of sale, which he will write up, that I can refer back to.
I'm not trying to collect personal data for other usage, but merely have done this in the past, whith no objection from either party, to cover us both.
Our state's law requires even private sellers to be aware of who they are selling to, and the burden is on the seller to make sure that it's not an out of state resident, and that the person is of legal age.
Nothing more, nothing less.