The stickiness of the real estate commission is a source of fascination for economists and curiosity for consumers who are doing an increasing share of the home-buying legwork themselves online. It also offers potential lessons for workers in other industries worried about the Internet’s destructive powers. The Web has changed how agents hustle for a share of the estimated $60 billion paid each year in residential real estate commissions. But it hasn’t taken their jobs. In fact, the number of agents has grown 60 percent in the past two decades.