The so-called sharing economy is supposed to offer a new kind of capitalism, one where regular folks, enabled by efficient online platforms, can turn their fallow assets into cash machines. According to its fans, Airbnb, along with the car-sharing company Uber, and others, is leading us into a less wasteful, more virtuous future. In it, anyone with excess time or space — or a car and a driver’s license — can easily become an entrepreneur with little to no start-up costs. But the reality is that these markets also tend to attract a class of well-heeled professional operators, who outperform the amateurs — just like the rest of the economy.