We analyze a model of Internet auctions. Sellers each offer one item of a heterogeneous good to bidders who have unit-demand preferences. Items are sold in second-price proxy auctions. We derive a perfect Bayesian (epsilon-) equilibrium. Our experimental findings support the theoretical results. An analysis of sellers' revenues in the Vickrey-auction reveals that they are non-monotonic in bids for substitutes valuations. We combine these results to investigate incomplete bidder-seller networks.