Facebook moves to restrict firearms sales
newsroom.fb.com
newsroom.fb.com
"For example, private sellers of firearms in the U.S. will not be permitted to specify “no background check required,” nor can they offer to transact across state lines without a licensed firearms dealer."
The former is implicit if you live in the same state as the seller, and it isn't a state that mandates checks for all sales. The latter is a felony, so banning it on Facebook just makes it congruent with the Gun Control Act of '68.
Note also that if you're not a Federal Firearms Licensee, it's illegal to make a business of selling firearms. Doing so on a public media like Facebook would seem to be particularly ill advised, not that the BATFE is very competent....