From online sources:
Google will show a Trusted Store badge next to the ads for the sellers shows the good track record and also take into account the reviews of Google Checkout, and so on.
When customers purchase from a Google Trusted Store, they can opt in to purchase protection from Google. If they opt in, the merchant will share order information and email address with Google, who will in turn send an email with the details of orders, merchant’s contact details, and link to contact Google in the event of an issue.
Note: It"s not officially and completely launched. Initially it starts only for limited number of retailers.