Although the Selling Partner API and Amazon Marketplace Web Service (Amazon MWS) are both web services that enable programmatic access to customer data, there are significant differences.
Here are some key differences between the Selling Partner API and Amazon MWS:
With the Selling Partner API you can develop applications for both sellers and vendors.
The Selling Partner API treats data as REST-compliant resources that can be accessed and modified via standard HTTP methods. By contrast, Amazon MWS exposes data using operations that are specific to Amazon MWS.
The Selling Partner API authorization leverages LWA, Amazon's implementation of OAuth 2.0. This model eliminates the need for the manual exchange of auth tokens, as required by Amazon MWS. See Authorizing Selling Partner API applications.
With Amazon MWS, selling partners authorize developers. With the Selling Partner API, selling partners authorize applications. Using the Selling Partner API, developers can create multiple applications that require varying levels of access to selling partner data.
The Selling Partner API provides finer grain data access control than Amazon MWS. Developers can request access to only the data they need, and selling partners can grant permissions at the API section, operation, or data resource level.
The Selling Partner API lets you directly procure and manage your own authentication credentials using AWS Identity and Access Management (IAM). With Amazon MWS, you receive authentication credentials provided by Amazon using a special registration workflow, and you get new credentials by opening a contact with Amazon MWS support. See Step 2. Create an IAM user.
The Selling Partner API uses AWS Signature Version 4 for authentication. Amazon MWS uses Signature Version 2. See Step 4. Create and sign your request.