The MWS team is pleased to announce the North America public launch of the Sellers API section in Amazon Marketplace Web Service. Previously, Sellers and developers had to manually determine in which marketplaces a seller was registered as well as information about listing in each marketplace (language, currency, etc.). The new Sellers API allows in one operation the retrieval of all marketplaces that a seller can sell in along with additional information about those marketplaces and the seller's participation in those marketplaces.
Note that the Sellers API is for determining what marketplaces the seller who is submitting the request is registered in. You cannot use this API to determine what marketplace other sellers are in, just your own marketplaces. More details about the new API can be viewed on the Sellers API section page of the MWS Developer portal.