Amazon Marketplace Web Service (MWS) provides data exchange support for listings, orders, payments, reports, and more. For listings, as an example, you can add, delete, and modify, and you can update price and quantity with a simplified feed. Listing, order, and payment data can be integrated into your existing workflows and business practices.
There is no additional fee for using Amazon MWS but you do need at least one of these Amazon MWS-enabled account types, which do have fees:
The Amazon MWS portals contain client libraries and comprehensive documentation. Take a look around: