The sharing economy. The collaborative economy. The Uber economy. It goes by many names. Whatever you call it, the phenomenon of specialized on-line marketplaces connecting small businesses with customers is growing by leaps and bounds. The primary builders of these marketplaces are tech-savvy, venture-capital-fed firms like Uber and Lyft (taxilike transportation services), Airbnb and Homeaway (short-term residential rentals), Instacart (grocery delivery), Lending Club (small-scale finance), and TaskRabbit (household errands). In theory, a reliable and comprehensive tax system should be readily able to adapt to a changing economy. But our tax system is held together with duct tape and clings to 20th-century business models. So the disruption to markets caused by the rise of the sharing businesses is poking holes in the existing tax apparatus.