I believe his original vision was to be the worlds largest book store offering. Something brick and mortar book stores couldn't do because of shelf space limitations.
Books weren't the end goal. He named it Amazon because he wanted something generic to expand into. I don't think third party sellers were in the initial vision but selling other items was
That was the slogan when it came out to steamroll books.com (the first mover), an outfit from Ohio that sold books on the Internet and shipped without plastic packaging in 1991.