29 septembre 2025, 14h15–15h30
Salle Auditorium 4
Industrial Organization seminar
Résumé
Many platforms charge percentage commissions on third-party sales. They may also charge per unit fees on ancillary services (such as FbA for Amazon's shipping and warehousing), and they may authorize (or be obliged) to offer seller opt-out of such services. We analyze the economics of these options and obligations. We derive and leverage a version of the Lerner equation to describe platform equilibrium pricing of ancillary services as a function of its commission rate and product demand elasticity. The formula readily extends to describe pricing when seller opt-out is available, and incorporates the elasticity of alternative supply. We show that ancillary services are priced higher under tied sales than opt-out than platform cost when commission rates exceed a threshold, so then the platform uses the ancillary good as a revenue source. Sellers and buyers then suffer from the tying of ancillary services. These results completely reverse when the commission rate exceeds the threshold. The ancillary good is then subsidized in order to mitigate third-party pricing magnification effects of double marginalization due to the commission rate inflating effective marginal cost. In this case, the platform has no incentive to tie ancillary services, but sellers and consumers may benefit from tying. With these insights, we then endogenize the commision rate. We show that it always induces a subsidy for ancillary services. When the regulator can control pricing of ancillary services (but not commission rates), we show that it has no reason to do so if its objective is maximal consumer surplus (platform incentives are fully aligned with consumers), but seller surplus is maximized by raising ancillary price to marginal cost! Conversely, capping commission rates (when the regulator cannot control ancillary prices) improves both consumer and seller surpluses.