On Efficient Trading Mechanisms with Ex-Post Individually Rational Traders

This paper studies a bilateral trading setting where the two agents are not ex-ante identified, in the sense that each of them may end up being a net buyer or a net seller. We derive a sufficient condition that ensures the existence of an (ex-post) efficient, (ex-post) budget balanced, (interim) incentive compatible trading mechanism that always yields a positive net utility to all agents (ex-post individually rational). This result improves a former existence result based on interim individual rationality showing that the stronger requirement of ex-post individual rationality does not always rule out efficiency.