I agree in general with the other commenter that it’d be difficult to do it systemically, which is why it should fall back to moderation.

Valve could say they don’t like gambling, make that a policy, and then say anyone caught doing so will lose their inventories or somesuch, then hire a team of people to sift through Steam marketplace trade data and identify gambling transactions for punishment. It would cost Valve money and wouldn’t end gambling by a long shot, but the goal would be to try to destroy the highest profile accounts who cause new players to want to gamble.

Basically instead of sitting on their hands and getting money, they could … try a little.