There are 2 ways to get your Amazon Seller ID. Below are detailed steps for each.
FIND YOUR SELLER ID ON YOUR AMAZON SELLER CENTRAL ACCOUNT
1. On your Amazon Seller Central Account, hover over to Settings and click on Account Info.
2. On your Account Info, click on Your Merchant Token.
3. Copy your Amazon Seller ID.
FIND YOUR AMAZON SELLER ID ON ANY OF YOUR PRODUCT LISTING
1.Go to any of your product listings.
2. Click on your company name.
3. Click the link to your storefront.
4. You can get your Amazon Seller ID on the address bar beside the word "merchant".