The building stands 600ft (183m) above sea level just below Garn Fawr
A former u-boat listening station on the Pembrokeshire coast that monitored submarine transmissions during World War II is up for sale.
In more recent times the red brick building has been used as a farm machinery workshop and store.
Prospective buyers have until Wednesday to submit sealed tenders with offers invited over £45,000.
It is one of a pair near the Strumble Head peninsula close to Fishguard. The other is now used by birdwatchers.
The building belongs to farmer John Davies and comes with close to an acre of land.
He said: "It is an unusual building but most of the Pembrokeshire coastline is littered with various types of World War two structures.
"My father came here to farm in 1946 and some years later once the Ministry of Defence had finished with it they returned it to the land it originally belonged to."
The brick building looks out over Strumble Head Lighthouse
He said he remembered it coming with virtually all of the equipment still inside including generators and various listening devices but that they have long since gone.
It is about 600ft (183m) above sea level where the other stands.
"They were used to listen in on u-boat transmissions. I assume the messages were in code and passed on to Bletchley Park.
"It worked in conjunction with the other building and from the height difference they were able to work out some sort of location."
For many years it housed the sea to shore radio transmitter used by the ferries crossing from Fishguard to Ireland but they now communicate via satellite.
Mr Davies said he currently used the building to store and work on farm machinery.
"I'm not doing as much farming these days and over the years numerous people have approached me to ask me about selling it," he added.
Estate agents JJ Morris, which is handling the sale, said there had been a great deal of interest in the building.
It is situated in the Pembrokeshire Coast National Park which has strict planning rules.
In 2004, a small brick shed overlooking the Pembrokeshire coast, once a telephone exchange and measuring just 15ft by 9ft, sold at auction for £21,000.