If your dealer has held the gun overnight without booking it in his records, he's probably already in Dutch with the BATF@E. All my local dealers require a picture ID to book a transfer gun in and threaten to refuse delivery and give it back to UPS if they don't get the ID. My individual sellers have always backed down and faxed the ID rather than have the gun placed in 'Common Carrier Purgatory' like that.
My take from what I've read and what the dealers have told me is that the BATF&E agent has advised them that if they're going to handle private party transfers, they better be sure whose ID they are booking the gun in under. The DL/PIC ID seems to be a 'safe harbor' for the dealer. Remember, the dealer is a private business man who is being watched by a non-friendly beaurocracy enforcing very fuzzy regulations. If he says this is how he wants to do business, its pretty much his call.
I recently sold two BRNOs to a poster here from Maine who wanted to ship through Kittery Trading Post. Kittery flat refused to accept a gun from me as an unlicensed individual no matter what ID I provided, so I had to ship through my local FFL. Maybe if your seller won't verify his identity, he can have his local dealer fax a FFL to ypour dealer and they can book the gun in through them...Geo