In addition to what Ryan mentioned, you have to remember that Amazon started from a very narrow niche. They did not start as the global e-commerce destination they are today, but rather from enabling people to buy books online. And only books. This gave book publishers and even bookstores with an additional channel through which to sell their products and consumers a new way to buy books. So there was both a great new, revenue-stream for sellers (and still is today) as well as a new, convenient way to buy for buyers (and still is today).