  Ad exchanges are an emerging platform for trading advertisement slots on the
web with billions of dollars revenue per year. Every time a user visits a web
page, the publisher of that web page can ask an ad exchange to auction off the
ad slots on this page to determine which advertisements are shown at which
price. Due to the high volume of traffic, ad networks typically act as
mediators for individual advertisers at ad exchanges. If multiple advertisers
in an ad network are interested in the ad slots of the same auction, the ad
network might use a "local" auction to resell the obtained ad slots among its
advertisers.
  In this work we want to deepen the theoretical understanding of these new
markets by analyzing them from the viewpoint of combinatorial auctions. Prior
work studied mostly single-item auctions, while we allow the advertisers to
express richer preferences over multiple items. We develop a game-theoretic
model for the entanglement of the central auction at the ad exchange with the
local auctions at the ad networks. We consider the incentives of all three
involved parties and suggest a three-party competitive equilibrium, an
extension of the Walrasian equilibrium that ensures envy-freeness for all
participants. We show the existence of a three-party competitive equilibrium
and a polynomial-time algorithm to find one for gross-substitute bidder
valuations.
