A terrorist hoping to buy an antiaircraft weapon in recent years needed to look no further than Facebook, which has been hosting sprawling online arms bazaars, offering weapons ranging from handguns and grenades to heavy machine guns and guided missiles.

The Facebook posts suggest evidence of large-scale efforts to sell military weapons coveted by terrorists and militants. The weapons include many distributed by the United States to security forces and their proxies in the Middle East. These online bazaars, which violate Facebook’s recent ban on the private sales of weapons, have been appearing in regions where the Islamic State has its strongest presence.

This week, after The New York Times provided Facebook with seven examples of suspicious groups, the company shut down six of them.