The general procedure is as follows;
Firstly, make sure the domain is unlocked in the Domains -> Registrations section of the panel. You will also find the auth code for the domain in Domains -> Reg.Transfer, make a note of this, it may be needed.
Also, make sure the contact details in the domain's WHOIS are current and valid.
Now, tell the buyer to initiate a transfer through their registrar. You should then receive an email allowing you to approve the transfer, this email will contain instructions on what you need to do.
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