The video game Counter-Strike: Global Offensive, in which players combine terrorists and rebels and shoot each other, is a favorite among professional esports. A tournament in early April at the Nationwide Arena in Columbus, Ohio, home of the NHL Blue Jackets, sold out and generated 71 million online views over four days. In May, TBS and WME/IMG will launch their own CS:GO league, as the game is called, streaming online games and television on Friday nights.

The game’s recent success makes it easy to forget that CS:GO wasn’t an instant 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 shooters to choose from. That all changed when Valve introduced something new: decorative virtual weapons called “skins” that can be purchased in-game and sold for real money.

In-game purchases aren’t new, but cash trading is Valve’s change. In two years, the number of CS:GO players has grown 1,500 percent. Today, at any given time, 380,000 people worldwide are playing the game.

When introducing the skins, Valve said in the Snipeguns article that the online gun bazaar will allow Counter-Strike players to “experience all the excitement of the black market arms trade without going through any of them living in the dark arsenal.” around stabbed. It was meant to be a hoax. But the references to the black market are very clear.

Reasonable people can debate whether competitive video games are a sport, but they have at least one thing in common with soccer, basketball, and soccer: people like to bet on the outcome. For CS:GO, the introduction of skins has caused the gambling market to thrive. People buy skins to earn money and then use those skins to make online bets on professional CS:GO games. Since there is a liquid market for turning guns or knives into money, betting on skins is essentially the same as betting for real money.

The popularity of CS:GO skyrocketed along with the gambling skin market. According to research firm SuperData, Valve has sold 21 million copies of the game since its debut nearly four years ago, bringing in a total of $567 million in revenue from the title. A number of sites now allow players to trade skins, sell them, or use bets.

This type of bet is far from mainstream, but a lot of people get it. According to research firm Eilers & Krejcik Gaming, in 2015 more than 3 million people bet $2.3 billion worth of skins on esports game results. This also contributes to Valve’s profits. The gambling site runs software created by Valve, and when the CS:GO skin is sold, the game developer gets 15 percent of the money.

“Counter-Strike isn’t about gaming anymore,” said Moritz Maurer, head of eSports integrity at gaming organization SportIM. “It’s all about betting and winning.”

Unregulated sports gambling is illegal almost everywhere in the world, and some lawyers say it is legal. The highest level of competition in professional gambling has seen match-fixing by people who want to win at gambling. Valve has seen scammers on occasion, but has not been discouraged from betting on its games, nor has it made any efforts to discourage people from using its product as currency. The company did not respond to repeated requests for comment on this story.

Counter-Strike gambling appeals to the game’s biggest fans: teenagers. Sven, a 16 year old Dutchman, is typical. He and his friends play CS:GO and watch the pros online. Sven said he first tried betting on skins after a friend told him that people were making tens of thousands of dollars from them, and his interest in CS:GO changed. “Since I bet, I play less,” he said. “You are so excited and hope your team wins. With every kill they make, every round they win, it makes you happier.”

Sven bets on an independent site called CSGO Lounge, which is the biggest destination for Counter-Strike skin bettors. Offers little information about the owner, how to comply with local gambling laws, or if any consumer protection measures are in place. Sven was not impressed. After all, CSGO Lounge has one of the biggest names in video gaming around the corner. “I have complete faith in this site because everyone in the community uses it,” he said. “It’s even recognized and endorsed by Valve.”