Reach out to Amazon Seller Central
The current business owner should reach out to the Amazon Seller Central team to authorise the new Seller Central account to sell his brand.
If the account is located in the US, the following form can be used: Amazon Brand Registry Brand Owner Authorization Letter (Must be logged into Seller Central to view).
Send inventory to the buyer
Once the buyer has the ability to add the products for sale, the seller should send all inventory included in the sale to the buyer.
Buyer lists inventory
Once the buyer has received the inventory in their Amazon account, they can then list the product for sale on their Store. The seller should then remove the offering from their account, leading to the buyer receiving the buy box for the product.
Transfer brand registry to the buyer
The seller should work with Seller Central to transfer the brand registry across to the buyer. This should include the authorisation letter above.
The buyer and seller need to work with Amazon to make sure all of the above goes through smoothly. Meaning they will both need to take action to make the transfer a success.