If you think of it in terms of the 1 or 2 pawn shops you might have access to, eBay's initial audience of 100 or 1000 as well as the usenet groups Omidyar would post to provides significant "single user" utility that most marketplaces can't provide at that same audience size. So while you're correct that it's not truly "single" user, eBay did provide significant value to the very very early sellers.
