The video game Counter-Strike: Global Offensive, in which players form teams of terrorists and counterinsurgents and shoot at one another, is a favorite of the professional e-sports circuit. A tournament in early April sold out Nationwide Arena in Columbus, Ohio, where the NHL’s Blue Jackets play, and generated 71 million online views over four days. In May, TBS and WME/IMG will launch their own league for CS:GO, as the game is called, streaming games online and broadcasting them on TV on Friday nights. ... The game’s current success has made it easy to forget that CS:GO wasn’t an immediate hit for game maker Valve. It was the latest in the aging Counter-Strike series and came out at a time when there was no shortage of other shoot-’em-up games to choose from. Everything changed when Valve introduced something new: decorative virtual weapons, known as “skins,” that could be acquired in the game and sold for real money. ... In-game purchases weren’t new, but the cash trade was Valve’s special twist. Within two years, the number of people playing CS:GO had grown 1,500 percent. Today, there are 380,000 people around the world playing the game at any given time. ... People buy skins for cash, then use the skins to place online bets on pro CS:GO matches. Because there’s a liquid market to convert each gun or knife back into cash, laying a bet in skins is essentially the same as betting with real money. ... By one estimate, more than 3 million people wagered $2.3 billion worth of skins on the outcome of e-sports matches in 2015. ... The best way to get players deeply engaged in games, the company had determined, was to give away virtual items of random value and encourage a robust market to trade them.