Setting a default marketplace will ensure that that marketplace will be shown when you sign in to your Seller Labs account. If a default marketplace is not selected, the marketplace you will see when you login is the first active marketplace listed in the Marketplace Connections page.
You can only set a marketplace as your default marketplace if that marketplace is active.
To set your default marketplace follow these steps:
1. Login to your Seller Labs account
2. From the left navigation click on Connections
3. Click on the circle under Default of one of your active marketplaces