"This is something that could greatly get out of control very quickly," King said.
In crafting the new policy, Facebook had to confront a patchwork of gun laws around the country. New York, for example, has some of the nation's toughest gun laws. It prohibits the sale of weapons such the AR-15 semi-automatic rifle, and it is one of 16 states that require background checks on buyers making private firearm purchases.
Under the new policy, Facebook would allow a user to list an AR-15 as long as it wasn't offered for sale in states where the weapon is illegal.
Also, Facebook will remove any posts in which a gun seller offers to skip a background check, even if such checks aren't required in the seller's state. And it will delete offers to sell guns across state lines without the involvement of a licensed firearms dealer, something required under federal law.
"This is one of many areas where we face a difficult challenge balancing individuals' desire to express themselves on our services and recognizing that this speech may have consequences elsewhere," Facebook said.
The company already has systems in place to remove advertising that is false and deceptive, and it prohibits ads for illegal drugs, tobacco and prescription drugs.
Dan Gross, president of the Brady Campaign to Prevent Gun Violence, complained that Facebook didn't go far enough.
"They are talking about a community-based reporting system. Do what these other companies did and shut it down. Shut down the private sales of guns," he said.
There's no way to know how many guns are sold via Facebook, because the transactions are actually completed offline, said John Feinblatt, chairman of Mayors Against Illegal Guns. But such sales have occurred.
In Kentucky, for example, federal authorities in February charged an Ohio man with illegally selling a 9 mm pistol to a Kentucky teenager in a transaction arranged through Facebook.
Feinblatt said that there are "virtual gun shows" online. His group issued a report in December showing 66,000 active ads on a popular gun sales website called Armslist.
