Amazon.com Inc.’s purchase of Whole Foods Market Inc. could fill an important logistics gap for the online retailer, providing hundreds of stores perfectly positioned to serve as pickup and delivery hubs for urban customers. Adding the more-than-430 stores in Whole Foods’s network would expand Amazon’s current neighborhood network by a factor of at least five, logistics experts said Friday. “It’s going to give them little warehouses all around the country in some of the big markets,” said Michael Knemeyer, a professor of logistics at Ohio State University’s Fisher College of Business.