Marginal Mechanisms For Balanced Exchange

We consider the balanced exchange of bundles of indivisible goods. We are interested in mechanisms that only rely on marginal preferences over individual objects even though agents' actual preferences compare bundles. Such mechanisms play an important role in two-sided matching but have not received much attention in exchange settings. We show that individually rational and Pareto-efficient marginal mechanisms exist if and only if no agent ever ranks any of her endowed objects lower than in her second indifference class. We call such marginal preferences trichotomous. In proving sufficiency, we define mechanisms, which are constrained versions of serial dictatorship, that achieve both desiderata based only agents' marginal preferences. We then turn to strategy-proofness. An individually rational, efficient and strategy-proof mechanism-marginal or not-exists if and only if each agent's marginal preference is not only trichotomous, but does not contain a non-endowed object in her second indifference class. We call such marginal preferences strongly trichotomous. For such preferences, our mechanisms reduce to the class of strategy-proof mechanisms introduced in Manjunath and Westkamp (2018). For trichotomous preferences, while our variants of serial dictatorship are not strategy-proof, they are truncation-proof and not obviously manipulable (Troyan and Morrill, 2020).