Valve Faces Lawsuit Over Video Game Gambling
bloomberg.com
bloomberg.com
Of the two people I know who play CS:GO neither play for the matches, but only use it to gamble. It's worth noting Valve isn't the first or only game to have these illegal markets built around them [1] but it's by far the largest.
IMO Valve has no responsibility here, as you can use anything as a currency. The government can't stop you from trading baseball cards.
From a brief read around though I couldn't find any successful suits.