This blog post explains how to test SP-API calls using selling partner sandbox endpoints.
Selling Partner APIs (SP-APIs) provide sandbox environments to test the SP-API endpoints without affecting production data. Directing calls to the SP-API sandbox endpoints allows you to make static requests that return mocked responses. By using the sandbox, you can learn the steps required to make a call, understand the request and response formats, practice validating application credentials, and play with API endpoints without impacting production data.
Read the full article on the Selling Partner API Blog.