Yes, External Transfer Service is the correct option, because it was not listed and you directly found the buyer; do not list it on Sedo.
Escrow.com is cheaper and faster, but only handles the financial side - they don't hold the domain during the transaction. For most escrows, this is sufficient security, but is not as safe as using services that hold both the money and the domain. Personally, I prefer escrow.com, but Sedo's escrow service is fine too.
Ron