I think he means from his GD account to the buyers GD account?
If so,
1. Unlock the domain
2. Click on "Change Account" (located under 'Manage Domains').
3. Fill in the buyers' details in the "new registrant" fields, check the agreement box, and click 'Save Changes.'
4. Buyer will receive an email to complete the change of accounts.
If your buyer wants to transfer out of GD, refer to the above post.