This paper examines eBay coin auctions to explore bidding and selling behavior, focusing on three main issues: the winner's curse, the cost of entry, and the choice of reserve prices. The authors find that a bidder's expected profits decrease by 3.2% when the expected number of bidders increases by one, indicating a significant winner's curse effect. They also document that bidders require an expected profit of $3.20 to enter an auction, highlighting the costliness of entry. Additionally, they study the seller's choice of reserve prices, finding that high-value items tend to use secret reserve prices with low minimum bids, while low-value items use posted reserve prices. This behavior is consistent with maximizing behavior. The paper also develops new techniques for structurally estimating common value auction models, providing insights into the dynamics of online auctions.This paper examines eBay coin auctions to explore bidding and selling behavior, focusing on three main issues: the winner's curse, the cost of entry, and the choice of reserve prices. The authors find that a bidder's expected profits decrease by 3.2% when the expected number of bidders increases by one, indicating a significant winner's curse effect. They also document that bidders require an expected profit of $3.20 to enter an auction, highlighting the costliness of entry. Additionally, they study the seller's choice of reserve prices, finding that high-value items tend to use secret reserve prices with low minimum bids, while low-value items use posted reserve prices. This behavior is consistent with maximizing behavior. The paper also develops new techniques for structurally estimating common value auction models, providing insights into the dynamics of online auctions.