Normally, the buyer would set up an account at your registrar (goDaddy or whatever), and you would "push" the domain to their account. This is usually free of charge. Look for the free push feature in your account.
If the buyer wants you to transfer the domain to another registrar, you must wait 60 days after the domain is registered & then you can do a transfer (more complicated than a push). There are charges to transfer the domain to a new registrar, but you usually get an extra year's renewal, depending on the registrar's rules.
Hope this helps!