The title is a little misleading, because he's not actually worth $1.2 Billion, however, that's how much money passed through the website. It's like eBay, lots of money passes through it, however, they don't get it all. As I read in the below article, he has about 600,000 bitcoins which amounts to about $78million, which is still a pretty penny. But yeah, shit is cray cray.
http://topinfopost.com/2013/10/03/fbi-arrests-29-year-old-mastermind-of-billion-dollar-internet-drug-blackmarket