An Impossibility Result for Ex-Post Implementable Multi-Item Auctions with Private Values

We analyze ex-post implementable social choice functions for private-value and quasi-linear settings over restricted domains of preferences, the leading example being multi-item auctions (with either heterogeneous or homogeneous goods). Our work generalizes the characterization of Roberts (1979) who characterized ex-post implementability over unrestricted domains. We show that ex-post implementability for multi-item auctions (and related restricted domains) implies weighted welfare maximization, if the given function also satisfies four additional social choice requirements. The most significant requirement is similar to Arrow’s IIA condition, adjusted to the quasi-linear case, and we study its connection to various existing monotonicity properties. JEL Classification Numbers: C70, D44