The competition with eBay started before 1999, and 3rd party sellers was not a major part of the strategy nor was it really concerned with eBay (which is 1999 was literally NOTHING but an auction site).
Amazon had its own "auctions" site in the late 1990s which many people forget even existed (it's one of the few things that Amazon tried and failed at). Bezos knew that eBay was a problem as soon as they emerged, and worried that Amazon would never compete effectively against them. In many senses, he was right.
How do I know this? I worked with Bezos in the legendary "garage" in Bellevue, WA.
Amazon had its own "auctions" site in the late 1990s which many people forget even existed (it's one of the few things that Amazon tried and failed at). Bezos knew that eBay was a problem as soon as they emerged, and worried that Amazon would never compete effectively against them. In many senses, he was right.
How do I know this? I worked with Bezos in the legendary "garage" in Bellevue, WA.