Optimal mechanisms for an auction mediator

We consider a dynamic auction environment with a long-lived seller and short-lived buyers mediated by a third party. A mediator has incomplete information about traders' values and selects an auction mechanism to maximize her expected revenue. We characterize mediator-optimal mechanisms and show that an optimal mechanism has a simple implementation as a Vickrey auction with a reserve price where the seller pays to the mediator only a fixed percentage from the closing price.