Successful entrepreneurs often spot innovative opportunities that mere mortals overlook. They’ve got a knack for recognizing the potential of an innovative product or service. Or they develop an entirely new way of marketing things that already exist. But the vision to view a situation from a unique angle isn’t always innate. It sometimes sharpens after a career that’s more of a roller coaster ride than a rocket trajectory. Whatever their particular journeys, here are 4 specialist businesses that spotted a gap in the market.