Fifteen years ago, Jack Ma—the founder of China’s Alibaba—was laughed out of Wall Street. Now the world’s biggest investors are clamoring for a piece of what could be the biggest IPO ever. On Monday, Alibaba held the first of 100 meetings in 10 days, selling itself to potential investors for the first time. It all started in New York City, where 800 people piled into three big rooms at the Waldorf Astoria. One of those in the crowd was Michael de la Merced, reporter for DealBook, a financial news service for the New York Times.