Bear in mind that the approach Amazon took was during a time in which e-commerce was still just taking hold and was far from commonplace.

Of course, starting with your own products and building an audience of shoppers gives you some leverage in attracting sellers when you change to a marketplace environment. The downside is that in the short term your product mix won't be as robust, and you'll rely solely on your own efforts to promote your site.

In the early stages a few enticements come to mind:

1. Early mover - for those early sellers, all important reviews and ratings will stack up and give them a lead over later adoptees of the platform.

2. Additional exposure - if you don't impose exclusivity on your sellers, they stand to benefit from having additional promotion to customers they may not otherwise reach - ie while the upside may be questionable - there is no real downside to listing products on the site, assuming that the time burden of doing so is low for the sellers.