Uniform price auctions with a last accepted bid pricing rule

We model multi-unit auctions in which bidders' valuations are multidimensional private information. Under a natural constraint on aggregate demand we show that the last accepted bid uniform-pricing rule admits a unique equilibrium with a simple characterization: bids are identical to those submitted in a single-unit first price auction. The form of equilibrium bids suggests that last accepted bid uniform-pricing is a generalization of single-unit first-pricing: in both auctions winners pay the highest market clearing price. Contrasting the separating equilibrium of the last accepted bid auction, we show that equilibrium bids in pay as bid and first rejected bid uniform price auctions must pool information. Thus other common multi-unit auction formats cannot generalize single-unit first-pricing, in which equilibria do not pool information. The existence of a unique equilibrium implies that price selection may be an additional tool for avoiding the zero-revenue equilibria which exist in the first rejected bid uniform price auction. Finally, we show that equilibrium bids in our private information model are significantly flatter than in an analogous random supply model, suggesting that uniform price auction bids may not be as steep as commonly believed.