On Aug. 2, online sports gamblers wagered $7 million on a tennis match in Poland. Stunningly, the money favored 87th-ranked Martin Vassallo Argüello, even after the Argentine lost the first set. Suspicious that the fix was in, the Internet gambling site Betfair voided the bets and alerted the Association of Tennis Professionals. Those reservations seemed justified when top-seeded Nikolay Davydenko quit in the third set, citing an ailing toe. In the wake of that fishy match, multiple tennis players have admitted they’ve been asked to fix results. Along with exposing the seamy side of pro tennis, the scandal has also spotlighted the site that handled the action. Most Americans probably haven’t heard of Betfair, but it’s the biggest thing going in global gambling.

Betfair, which opened for business in 2000, is best described as day trading for sports bettors. Using Web-based accounts, anonymous users can set their own odds or bid on odds offered by other players. Online “betting exchanges”—there are dozens, but Betfair is the kingpin, with a 90 percent market share—eliminate the role of odds-setting middlemen like local bookies and Las Vegas sports books. Instead of wagering on take-it-or-leave-it odds set by the house, gamblers are free to choose among many different price points, striking bets for as little as $1 up to hundreds of thousands.

Now imagine betting on sports or playing casino games from your cellphone. No longer are you restrained to sitting in front of the computer you can play games or place wagers on the sport game you’re watching at the bar.