I think my question was answered, but let me clarify what I was asking:
I'm the typical "enthusiast" gun buyer. Every now and then I buy a gun from someone in another state. I pay for the gun in advance and then the seller ships the gun to my designated FFL. For about 8 years, I have been using the same FFL locally to receive the gun, run a NICS and do the 4473 so I can take possession of the gun. They never asked for the seller to send them a copy of their driver's license until about 2 years ago +/-. Now, they are adamant that they MUST have the Seller's DL to log the gun in their book and do the NICS background check on me.
I have never had a seller outright refuse to send them their DL, but about 1/2 the time they forget and I have to track down the Seller to do this before my designated FFL will complete the paperwork. One time, the FFL told me that the law requires them to get a copy of the Seller's ID. I lightly questioned him on this, but didn't push it.
So, I just wanted to know what is legally required; and if not legally required, then what the industry standard is on this. I thought that there was no federal law requiring a seller of a firearm to identify himself to a purchaser. In fact, with FTF transactions, I've never asked a seller to see his ID - I don't know what purpose it would serve anyway, since no one is running background checks on Sellers.