What happens to my WhatsApp account if my carrier sells my number to someone else?
Simple answer No you can't use that account anymore

Reason being that WhatsApp is linked to your number. If the carrier sells that number and new owner activates WhatsApp , they will be prompted to authenticate and a new account is created for them. If they don't install WhatsApp you are good to go to the next time Whatsapp asks for authentication

Actions to be taken for change of number before changing number are mentioned in WhatsApp FAQ