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Modeling online auctions with proxy-bidding: Ascending versus Sealed Model

This paper presents several empirical tests of the hypothesis that eBay bidders bid “as if” they are in a sealed bid auction. The tests rely on observations of the magnitudes and timing of top two proxy bids. We apply the tests to data from three eBay markets - MP3 players, DVDs and Chevrolet Corvettes. All tests reject the sealed-bid model in all three datasets, and a significant portion of auctions in each dataset is better described by an ascending-bid model. A detailed test assuming independent private values rejects the sealed-bid model even in an otherwise “plausibly sealed” subset of the data which does not conform to the ascending-bid model. Given these findings, demand-estimation using eBay data is more difficult than previously thought. In particular, the results suggest that the empirical strategies based on multiple order-statistics of the bidding distribution as in Song (2005) will not work.