John Wyllie, of Greenlaw Avenue, Paisley, touted the domain names after his plans for a kiosk-based holidays service in sub-post offices fell through.
Mr Wyllie said he had registered names, including postofficeholidays.co.uk, with the company's blessing.
But the national giant said it had never been involved in any contract with him.
He may have agreed the plan with other partners, but Post Office Ltd itself was not involved and expressed concerns about branding of the kiosks.
The company complained to internet dispute resolution service, Nominet, and an independent expert has now ordered that the domain names be handed over.