The domain name
is for sale!
Your note goes to the person who owns this domain.
Only its owner can publish a page on this domain.
It moves to the registrar you already use, and it's yours.
Send the owner a note from this page. They'll reply from their own email.
Settle the price and how to pay between you. Many buyers use an escrow service.
The owner moves the domain to your registrar account, and you're done.