This article outlines how to find your Amazon Seller ID, which you will need in order to set up your Amazon Selling Partner Integration.
Step 1: Amazon Seller Central
Log in into your Amazon Seller Central account.
Step 2: Go to Account Settings
In the upper right-hand corner, you will find a gear. Click on the gear and select "Account Info."
Step 3: Go to Business Information section
Find and click on "Merchant Token."
Step 4: Copy the Merchant Token
Copy the ID that is displayed. This is the Merchant Token, also called your Seller ID.