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Many platforms that facilitate transactions between buyers and sellers charge ad valorem fees in which fees depend on the transaction price set by sellers. Given these platforms do not incur significant costs that vary with transaction prices, their use of ad valorem fees has raised controversies about the effciency of this practice. In this paper, using a model that connects platforms' use of ad valorem fees to third-degree price discrimination, we evaluate the welfare consequences of banning such fees. We find the use of ad valorem fees generally increases welfare, including for calibrated versions of the model based on data from Amazon's marketplace and Visa's signature debit cards. [ABSTRACT FROM AUTHOR]

This paper investigates a puzzle and possible policy concern: Why do platforms such as eBay and Visa that enable the trade of goods of different unobserved costs and valuations rely predominantly on linear ad-valorem fees, that is, fees that increase in proportion to the sale price of the trades that they enable? Under a broad class of demand functions, we show that a linear ad-valorem fee schedule enables a platform to maximize its prot as if it could actually observe the costs and valuations of the goods traded and set a different optimal fee for each good. Surprisingly, we find for this class of demands, allowing the platform to set ad-valorem fees (i.e. price discriminate) increases social welfare, both when the platform is regulated to recover costs and when the platform is unregulated. [ABSTRACT FROM AUTHOR]

This online appendix contains the derivation of the closed form solutions for the examples used in the main paper. It also details the numerical analysis performed to check the robustness of the first part of Corollary 1 to the introduction of private benefits and spillovers.

In an increasing number of industries, firms choose how much control to give professionals over the provision of their services to clients. We study the tradeoffs that arise in choosing between a traditional mode (where the firm takes control of service provision) and a platform mode (where professionals retain control over service provision). The choice of mode is determined by the need to balance two-sided moral hazard problems arising from investments that only professionals can make and investments that only the firm can make, while at the same time minimizing distortions in decisions that either party could make (e.g. promotion and marketing of professionals' services, price setting, choice of service offering, etc.).