And yes, Merchant and Seller IDs are used interchangeably.
You can find your merchant token under Settings -> Account Info -> Under Business Information -> Your Merchant Token.
Merchant Token is not the same as merchant ID or Seller ID.
What is a merchant token on Amazon?
Merchant tokens are an old identifier and no longer used. They are not necessary for any Amazon system and should not be necessary for 3rd party applications. MWS only uses these identifiers: Seller ID (or Merchant ID)
What is my Amazon Seller ID?
In the upper left side of the page click the ‘View my Seller Profile’ link. Your storefront will appear. In the URL you will see, near the end of it, seller=. What is following that will be your SellerID.
What is a seller ID?
Your “seller ID” is your “user ID” when you are the seller. You “buyer ID” is your “user ID” when you are the buyer. Some users keep separate accounts for their selling and buying activities, so one person might have both a “seller ID” and a “buyer ID” but they are both still just their “user ID.”
Where do I find my Amazon MWS auth token?
To find the MWS Auth Token, log into your Seller Central account and navigate to the User Permissions page in the Settings. Scroll to the bottom of this page and locate the Third party developer and apps section, from here click on Visit Manage Your Apps.