This article outlines how to find your Amazon Seller ID which you will need in order to setup your Amazon Selling Partner Integration
Step 1: Amazon Seller Central
Login 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