Amazon and Amazon marketplace are both online stores from which buyers from any part of the world can purchase items. Amazon is the online company store itself in which the company goods are being sold. It is widespread and a commonly used site which offers a rating system for its products and descriptions of products, as well as information concerning the goods they have to offer.
Amazon marketplace is a collective group of all Amazon retailers whose products are put to sale in one place. Amazon.com can be compared to a retail shop where the owner of the shop is the one who sells them. Amazon marketplace is akin to a shopping mall, which is comprised of many retail shops put together in one place.