thats transfer if the buyer have account in same registrar then you can do a push which u can find in the domain panel with your registrar
The safest way for both you and the buyer is to use an escrow service. There are many providers. I have used www.escrow.com for domain purchase and everything went very smoothly. On their page they have the process explained step-by-step.
If its with godady they have a domain ownership transfer feature you will need the new owners account number and email I believe, then they need to accept ownership.