To sell an unused account, should be fine.
Although I'm not sure why anyone would want to buy an account from a third party, when they can buy directly from Blizzard or a retailer.

If you're selling an account which has been activated, that would definitely be against the terms of service.



You can send an RAF invite to any email address.
They can use the code, to create their account... which is then RAF linked to yours.
Or they can send the RAF invite to your email address, and then you're linked (with the new account) to their account.

Either way, once the account is created use your own retail/digital code to upgrade.
No need to sell an unused account between the two parties.