The Merlin Aerees 3.4 litre 2+2 drophead coupe was built by Dennis Rees, who decided in the Thirties that he could produce a vehicle just as exciting as the William Lyons Swallow sidecars of the time.
He formed the Aerees Motor Company of Carmarthen, in south Wales, to launch his high performance vehicle, but then the Second World War intervened.
After serving in the Royal Air Force, Mr Rees returned to Wales to start a garage business and the idea was forgotten until 1977 when his daughter persuaded him to revive it.
Using scrap metal for the body, Mr Rees finally completed the vehicle in 1984.
The engine and gearbox came from a Jaguar 3.4 Mk II and the body had sweeping wings reminiscent of the classic Jaguar Super Sports. The bonnet was made of panels from Frigidaires, the doors from Avery petrol pumps and the petrol tank from a Simca roof.
Malcolm Barbourn, auctioneer at Sotheby's, said: 'It's wonderful. A really nice, unique motor car and a classic in its own right.'
Mr Rees died earlier this year and his wife, Joan, 72, is selling the car and others he had collected. 'It's so sad to see it go. It was part of our lives and my husband loved it. I hope it will go to a good home and someone who will appreciate it,' she said.
Mrs Rees will also miss the little 1937 Fiat Topolino which she and her husband used in their courting days. It is estimated at pounds 1,000, while a 1954 Riley RME saloon and a 1965 Daimler 2.5 litre V8 saloon could fetch pounds 2,500 each.