Truthful Bilateral Trade is Impossible even with Fixed Prices

TL;DR

This paper extends the classical bilateral trade impossibility result to multi-unit and multi-good settings, showing that truthful mechanisms cannot guarantee more than a fraction of the optimal gain-from-trade, even with fixed prices.

Contribution

It demonstrates that the bilateral trade impossibility persists in multi-unit and multi-good scenarios, providing tight bounds on achievable efficiency.

Findings

No truthful mechanism can guarantee more than 1/M of optimal gain in multi-unit cases.

In multi-good scenarios, the guarantee is limited by the harmonic number H_M.

Bounds are proven to be tight, confirming the fundamental limitations.

Abstract

A seminal theorem of Myerson and Satterthwaite (1983) proves that, in a game of bilateral trade between a single buyer and a single seller, no mechanism can be simultaneously individually-rational, budget-balanced, incentive-compatible and socially-efficient. However, the impossibility disappears if the price is fixed exogenously and the social-efficiency goal is subject to individual-rationality at the given price. We show that the impossibility comes back if there are multiple units of the same good, or multiple types of goods, even when the prices are fixed exogenously. Particularly, if there are $M$ units of the same good or $M$ kinds of goods, for some $M\geq 2$, then no truthful mechanism can guarantee more than $1/M$ of the optimal gain-from-trade. In the single-good multi-unit case, if both agents have submodular valuations (decreasing marginal returns), then no truthful mechanism can guarantee more than $1/H_M$ of the optimal gain-from-trade, where $H_M$ is the $M$-th harmonic number ($H_M\approx \ln{M}+1/2$). All upper bounds are tight.