Can private companies legitimately regulate sharing markets, and if yes, how? Whereas scholars have either criticized sharing platforms for expanding into private and public arenas or welcomed them to counterbalance encroaching government regulations, studies document their unbridled popularity. On the basis of a special version of social contracts theory pioneered by James Buchanan, we develop a heuristics that helps guide reasoning about the legitimacy of the sharing economy’s regulatory function. First, we discuss the conditions under which free and responsible individuals deliberately subject themselves to rules without their prior direct participation, that is, exit, voice, and constitutional limits. Second, we suggest sharing platforms can use novel ordo-responsibilities to establish a sharing constitution that takes these conditions into account. Third, we argue that sharing platforms can legitimately do so within an enabling institutional environment in society, the provision of which relies on the joint efforts of sharing platforms, political actors, and civil society.