Tian Jia made the kind of returns on his money that investors in the rest of the world could only dream of

The 29-year-old Beijing-based programmer had $440,000 on deposit at the Hong Kong bitcoin exchange Bitfinex until last week and, on good days, would wake up to find a couple thousand more dollars in his account than when he went to sleep. The earnings came from lending his dollars to traders who wanted to leverage their bets.