Bilateral Trade with Multiunit Demand and Supply

Abstract

We study a bilateral trade problem with multiunit demand and supply and one-dimensional private information. Each agent geometrically discounts additional units by a constant factor. We show that when goods are complements, the incentive problem—measured as the ratio of second-best to first-best social surplus—becomes less severe as the degree of complementarity increases. In contrast, if goods are substitutes and each agent’s distribution exhibits linear virtual types, then this ratio is a constant. If the bilateral trade setup arises from prior vertical integration between a buyer and a supplier, with the vertically integrated firm being a buyer facing an independent supplier, then the ratio of second-best to first-best social surplus is, in general, not monotone in the degree of complementarity when products are substitutes and is increasing when products are complements. Extensions to profit maximization by a market maker and a discrete public good problem show that the broad insight that complementarity of goods mitigates the incentive problem generalizes to these settings. This paper was accepted by Joshua Gans, business strategy. Funding: Financial support from the Samuel and June Hordern Endowment, the University of Melbourne Faculty of Business & Economics [Eminent Research Scholar Grant], and the Australian Research Council [Grant DP200103574] is acknowledged.