Every seller on Amazon has a distinct Seller ID.
When you create an MWS Auth Token for Zonmaster the number will be displayed on the results screen. But if you didn't copy that screen (note: you should copy that screen!) you can find it in other ways.
Depending on your account type and how long you've been on Amazon try :
1. Your storefront. Accessible through your Seller Account page > Store Settings > View my storefront
Then look at the URL in the browser address bar .....
My URL is http://www.amazon.com/gp/shops/storefront/index.html?ie=UTF8&marketplaceID=ATVPDKIKX0DER&sellerID=ANZE9ULNDLIJJ
"Seller Id" and "Merchant Id" are the exact same thing. Whoever was coding on one side didn't tell the other side they were changing it. Even the new MWS API's have both of them. UGH!
2. Go to any product page where you have a listing. Put the mouse over your listing and you'll see ....
Right-click to copy either. Paste anywhere and copy your Id down. This is what you will need to sign up with Zonmaster.