In a stunning blow to a novel way to buy products and services, the world's largest exchange for trading bitcoin currency shut down Tuesday, triggering a massive sell-off and sending many prospective investors away — perhaps for good. "This is extremely destructive," said Mark Williams, a risk-management expert and former Federal Reserve Bank examiner. "What we're seeing is a lot of the flaws. It's not only fragile, it's fragile as eggshells." The mysterious circumstances that triggered the failure of the exchange, Mt. Gox in Tokyo, is only adding to the renewed anxiety over the virtual currency, which just a month...