Your Amazon Seller ID is also known as your Amazon Merchant Token
To find your Amazon Seller ID, log in to your Amazon Seller Central Account at https://sellercentral.amazon.com.au/home
Go to Settings -> Account Info on the top right or go to https://sellercentral.amazon.com.au/hz/sc/account-information
In the Business Information section click the Merchant Token link
Copy this token to use for your Omnivore login
If you cannot see the link to Merchant Token please check that you have a Professional Account for Amazon AU at Settings -> Account Info on the top right or go to https://sellercentral.amazon.com.au/hz/sc/account-information. You will not be given a Merchant Token if you have an Individual Account type