Foundations of Platform-Assisted Auctions

Today, many auctions are carried out with the help of intermediary platforms like Google and eBay. We refer to such auctions as platform-assisted auctions.Traditionally, the auction theory literature mainly focuses on designing auctions that incentivize the buyers to bid truthfully,assuming that the platform always faithfully implements the auction. In practice, however, the platforms have been found to manipulate the auctions to earn more profit, resulting in high-profile anti-trust lawsuits. We propose a new model for studying platform-assisted auctions in the permissionless setting. We explore whether it is possible to design a dream auction in thisnew model, such that honest behavior is the utility-maximizing strategy for each individual buyer, the platform, the seller, as well as platform-seller or platform-buyer coalitions.Through a collection of feasibility and infeasibility results,we carefully characterize the mathematical landscape of platform-assisted auctions. We show how cryptography can lend to the design of an efficient platform-assisted auction with dream properties. Although a line of works have also used MPC or the blockchain to remove the reliance on a trusted auctioneer, our work is distinct in nature in several dimensions.First, we initiate a systematic exploration of the game theoretic implications when the service providers are strategic and can collude with sellers or buyers. Second, we observe that the full simulation paradigm is too stringent and leads to high asymptotical costs. Specifically, because every player has a different private outcomein an auction protocol, running any generic MPC protocol among the players would incur at least $n^2$ total cost. We propose a new notion of simulation calledutility-dominated emulation.Under this new notion, we showhow to design efficient auction protocols with quasilinear efficiency.