When you sell a device how with serialnumber?

You registered the product with the serialkey on the product in your account.

If you sell the product what do you need to do to get it unregistered in your account so the new owner can register the product?

I'm assuming you mean the MPC-Live.

I don't know about now, but 3 years ago I sold my Live on ebay. It was cunningly simple to transfer the registration and content to the buyer.

Just go to your account at Akai and edit it by changing your name to the buyer's name, changing your email to the buyer's email and then create a password so that the buyer can access the account and can change the password on their own time.
Then I copied the link and sent it to the buyer as he waited for the Live to arrive. Boy, was he happy to get all that content onto his mac because he was fiending to make beats ASAP