Now why would the ATF suggest that?
"Each year, thousands of firearms are reported lost or stolen. The owners’ ability to adequately identify these firearms is central to law enforcement’s ability to investigate these crimes and losses."
http://www.atf.gov/pub/fire-explo_pub/p33128prsnl_fa_rec.pdf
Seems that it would provide some protection to the buyer and/or seller if the firearm was ever used in a crime.