This paper examines eBay coin auctions to explore online bidding and selling behavior, focusing on three main issues: the winner's curse, costly entry, and reserve price choices. The authors find that the winner's curse is significant, with expected profits falling by 3.2% when the number of bidders increases by one. Bidders require an expected profit of $3.20 to enter an auction, indicating costly entry. Sellers of high-value items tend to use secret reserve prices, while lower-value items use posted reserves. The study also develops new techniques for structurally estimating common value auction models. The paper discusses the role of reserve prices in eBay auctions, noting that high-value items are more likely to use secret reserves, which are not publicly known. The authors argue that this is consistent with maximizing behavior. They also find that the number of bidders is endogenous, meaning that bidders choose which auctions to attend, and that this affects the auction outcomes. The study uses a dataset of eBay coin auctions to analyze bidding behavior, finding that bidding activity is concentrated at the end of the auction, and that the number of bidders varies significantly. The authors also find that the number of bids submitted decreases as the ratio of the minimum bid to the book value increases. High-value items are more likely to be auctioned with secret reserves, while lower-value items use posted reserves. The paper also discusses the impact of reserve prices on bidder behavior, finding that high-value items are more likely to be auctioned with secret reserves, while lower-value items use posted reserves. The authors argue that the observed behavior of sellers appears to be optimal, as it maximizes their expected profits. The study concludes that the winner's curse is significant in eBay auctions, and that the number of bidders is endogenous, affecting auction outcomes. The authors also find that the number of bidders is a significant determinant of bid levels, and that the choice of reserve price policy affects the number of bidders who participate in the auction. The study provides new insights into the behavior of bidders and sellers in online auctions, and highlights the importance of reserve prices in determining auction outcomes.This paper examines eBay coin auctions to explore online bidding and selling behavior, focusing on three main issues: the winner's curse, costly entry, and reserve price choices. The authors find that the winner's curse is significant, with expected profits falling by 3.2% when the number of bidders increases by one. Bidders require an expected profit of $3.20 to enter an auction, indicating costly entry. Sellers of high-value items tend to use secret reserve prices, while lower-value items use posted reserves. The study also develops new techniques for structurally estimating common value auction models. The paper discusses the role of reserve prices in eBay auctions, noting that high-value items are more likely to use secret reserves, which are not publicly known. The authors argue that this is consistent with maximizing behavior. They also find that the number of bidders is endogenous, meaning that bidders choose which auctions to attend, and that this affects the auction outcomes. The study uses a dataset of eBay coin auctions to analyze bidding behavior, finding that bidding activity is concentrated at the end of the auction, and that the number of bidders varies significantly. The authors also find that the number of bids submitted decreases as the ratio of the minimum bid to the book value increases. High-value items are more likely to be auctioned with secret reserves, while lower-value items use posted reserves. The paper also discusses the impact of reserve prices on bidder behavior, finding that high-value items are more likely to be auctioned with secret reserves, while lower-value items use posted reserves. The authors argue that the observed behavior of sellers appears to be optimal, as it maximizes their expected profits. The study concludes that the winner's curse is significant in eBay auctions, and that the number of bidders is endogenous, affecting auction outcomes. The authors also find that the number of bidders is a significant determinant of bid levels, and that the choice of reserve price policy affects the number of bidders who participate in the auction. The study provides new insights into the behavior of bidders and sellers in online auctions, and highlights the importance of reserve prices in determining auction outcomes.