we develop a tractable model of a platform, like an app store, where heterogeneous sellers and consumers interact. sellers vary in quality and face entry costs, while consumers differ in willingness to pay for quality and usage intensity. the platform sets a consumer participation fee and an ad valorem commission on sellers. we characterize the equilibrium allocations and fees, and illustrate a key ratio of seller revenue to consumer surplus governing distortions in seller and consumer participation. we study the implications of cap regulations on commissions, allowing an own-payment channel for sellers, and the platform selling its apps.