Go to security settings on the one you want to sell and do a factory reset. That will remove all your apps and account information and return it to the way it was when you took it out of the box. The when you sell it the new owner will either sign in with their Gmail account or create a new if they don't already have one. Yours will no longer be on that device. That's the easiest way.
Romans 1:16 - I am not ashamed.