A few years ago BATF put out a letter emphasizing that the FFL executing a transfer needs to be sure that they know who is supplying the firearm. Some FFLs quit accepting guns from non-licensees, some would only accept if the seller showed up in person, and some would only accept if the seller included a copy of an ID. It isn't a BATF reg, per se, but the bureau has made it clear that it is the FFL's responsibility to comply, without spelling out what constitutes compliance.