A Georgia gun dealer was hit with an eight-year prison sentence this week for trying to sell an unregistered anti-tank rifle that he advertised on Instagram.

Mark Mann is the owner of a licensed gun dealership, The Rifleman, in the city of Macon, but the weapon he posted for sale on the photo-sharing service one day in 2014 was decidedly illegal. The gun he was looking to unload was a vintage Finnish model called a Lahti, and must be registered with the Bureau of Alcohol, Tobacco, Firearms and Explosive as a “destructive device” under the National Firearms Act, which Mann had not done. Federal prosecutors won a conviction against Mann in April.

The case is fresh fodder for the long-running controversy over gun sales on popular social networks. Some tech companies have tried to address the issue by banning gun sales altogether. Craigslist prohibits any gun or ammunition sales regardless of whether the seller has a license to deal firearms, while online auction hub eBay also bans guns and ammo, including dummy or inert rounds. Facebook, which has owned Instagram since 2012, has tried to strike a balance, but found itself struggling to set and enforce a consistent policy as a result.

Last year, Facebook settled on banning all gun transactions between private individuals, which are legal in most states but, unlike licensed sales, do not involve a background check for the buyer. Facebook’s policy extended to so-called kitchen table dealers, who are licensed to sell guns but don’t operate brick-and-mortar stores. The social network does allow gun selling by licensed gun retailers, explicitly condoning those posts “as long as all applicable laws and regulations are followed.”

Instagram has what appears at first glance to be an even stricter policy. The photo-sharing network’s community guidelines page says that “buying or selling firearms” is barred, “even if it’s legal in your region,” full stop. Yet many licensed dealers maintain Instagram accounts where they advertise sales. Take Bud’s Guns, a Tennessee dealer with a well-known mail order business. Bud’s Instagram page highlights deals on weapons like Kahr pistols, linking to the shop’s e-commerce website.

Facebook did not immediately respond to a request for comment on whether the company vets posts by licensed dealers to ensure sellers are operating lawfully. As of this writing, the Rifleman post advertising the illegal anti-tank rifle was still up, along with several others advertising rare firearms.

Policing the gun sellers — authorized, and otherwise — who use Facebook’s platforms could be beyond the capacities of its in-house monitors. After all, the federal government has an entire law enforcement and regulatory bureau devoted to inspecting licensed dealers and making sure those stores comply with laws. That would be the ATF, which, though able to stop Mann’s caper, only manages to inspect about 7 percent of gun sellers annually.