Gun Sellers Are Sneaking Onto Facebook’s Booming Secondhand Marketplace

An online gun bazaar is taking shape on Facebook Inc. FB -0.77% ’s Marketplace, as recent mass shootings have renewed the debate in Washington over access to firearms.

Facebook launched its Marketplace feature four years ago, allowing its more than two billion users to buy and sell almost any secondhand item by clicking a button on their home page. The private sale of many items, including guns, is specifically forbidden by Facebook rules.

But sellers are getting around that with a simple trick: They list gun cases or boxes at...