When second-price auctions have been conducted in the laboratory, most of the observed bids have been "overbids" (bids that exceed the bidder's value) and there are very few underbids. Few if any of the subjects in those experiments had any prior experience bidding in auctions. We report on sealed-bid second-price auctions that we conducted on the Internet using subjects with substantial prior experience: they were highly experienced participants in eBay auctions. Unlike the novice bidders in previous (laboratory) experiments, the experienced bidders exhibited no greater tendency to overbid than to underbid. However, even subjects with substantial prior experience tend not to bid their values, suggesting that the non-optimal bidding of novice subjects is robust to substantial experience in non-experimental auctions. A key determinant of bidding behavior was whether a subject had ever been a seller on eBay.