This excerpt taken from the AMZN 10-K filed Feb 16, 2007.
Amazon.com Retail and Third-Party Sellers
We source and sell a broad range of products to our customers worldwide across dozens of product categories. Through Amazon Services, we offer Amazon Marketplace and Merchants@ programs that enable third parties to sell their products on our websites, allow customers to shop for products owned by third parties using our features and technologies, and allow individuals to complete transactions that include multiple sellers in a single checkout process. We are not the seller of record in these third-party seller transactions, but instead earn fixed fees, sales commissions, per-unit activity fees, or some combination thereof.
Amazon Marketplace generally serves individuals and small businesses, enabling them to offer their products for sale on our websites alongside our products and products made available by other third-party sellers. Our Merchants@ program generally serves larger, branded businesses and is primarily focused on expanding the selection of new products available on our websites. We also offer fulfillment-related services to third-party sellers through Fulfillment by Amazon.
Our retail and third-party seller businesses relentlessly focus on customer experience by offering our retail customers a wide selection of merchandise, low prices, and convenience.