Facebook’s decision last week to prohibit private gun sales on its social networking site and on its photo-sharing app Instagram should make it harder for convicted felons and other dangerous people to acquire deadly weapons. That’s a very good thing, especially considering the unwillingness of lawmakers in Congress to enact sensible gun safety policies.

In recent years, Facebook has become a thriving marketplace where people buy and sell all kinds of goods and services, including firearms. People often post ads to buy or sell guns on the site and meet in person to complete the transaction. But unlike licensed dealers, private sellers are not required to conduct criminal background checks in most states, and Congress has refused to close this gaping loophole at the federal level.

It’s hard to say exactly how many posts offering to sell or buy guns exist on Facebook at any given time. But one recent study estimated that Facebook was the second most popular site for gun ads in the state of Nevada.That study was by Everytown for Gun Safety, a research and advocacy organization backed by Michael Bloomberg. Some of the guns sold on Facebook have been used in crimes. In December 2014, for example, Brian Harleman, an Ohio man, shot and injured a former girlfriend, killed her 10-year-old daughter and committed suicide using a gun he bought through Facebook. If he had tried to buy the gun at a dealer he would have been denied because of an earlier felony conviction.

Other marketplace websites like Craigslist and eBay have long banned gun sales. Facebook has been a bit slower to adopt this policy in part because it came later to the world of person-to-person commerce — it only started letting users transfer money to each other last year.