Bill Ackman, the billionaire stock player of the moment went bust on JC Penney and sold his shares for a $500 million loss this week. Citigroup Inc. managed the 39.1 million shares of the Plano, Texas-based department-store chain at a price of $12.90 apiece—a sale that ends a saga in which Ackman handpicked CEO Ron Johnson’s plan to turn the chain’s stores into collections of boutiques alienated customers, leading to the worst annual sales in more than two decades. It also follows Ackman’s agree...