eSports game developer Valve Corporation says it will no longer permit third-party gambling sites to make use of its Steam marketplace.

In a notice posted to the Steam site on Wednesday, Valve’s Erik Johnson wrote that using the eSports marketplace “to run a gambling business is not allowed by our API nor our user agreements.”

Johnson said Valve, the developers behind games like CounterStrike; Global Offensive (CS:GO) would “start sending notices to these sites requesting they cease operations through Steam, and further pursue the matter as necessary.” Johnson warned players to “consider this information as they manage their in-game item inventory and trade activity.”

Johnson stressed that Valve had “no business relationship” with these third-party gambling sites and had “never received any revenue from them.” Johnson noted that Steam “does not have a system for turning in-game items into real-world currency.”