The collection - more than 1,400 pieces of Louis XV and Italian furniture, porcelain, silver and paintings - belonged to Guiseppe Rossi, a Turin antique dealer who could not bring himself to sell any of them.
His treasures were stored in rented apartments across the city, where the only regular visitors were Mr Rossi, his sister, Maria Luisa, and a woman who helped them dust the collection every two or three days.
Mr Rossi died in 1989. Now his sister is selling the collection to fund a school to train people in the art of restoration and the conservation of antiques.
Mario Tavella, who catalogued the furniture for Sotheby's, said it was the biggest private collection he had seen. "It was like walking into Aladdin's cave," he said. "Everything was in fantastic condition and had obviously been so well looked after.
"[Mr Rossi's] whole life was the collection. But he did not build it up as a status symbol, it was just for the love of the fine furniture and to be able to look at it every day."
Laura Russo, of Sotheby's Turin office, used to visit Mr Rossi in his gallery. In the four-volume sale catalogue, she writes: "Under his severe appearance he had a velvet soul and an exquisite taste for art. The objects collected were the great and only aim of his life, apart from his profound attachment to his sister.
"The proceeds [from the sale], according to his wishes, will be used for noble and generous purposes, which is exactly as Mr Rossi was in his life."
The collector was born in 1914 to a family of craftsmen in Turin. At 15 he worked in his father's shop until he read a book which had a dramatic impact on him. The book, Frammenti di Vita, by an art collector, Riccardo Gualino, showed how a man with intelligence might find success through study and a willingness to learn.
Mr Rossi enrolled in university, graduating in business in 1937 and taking a job with the largest paper manufacturer in Italy. His enthusiasm and dedication brought rapid promotion but he was called up in 1942, working as a translator. After the war, he expanded his father's business and opened an art gallery. For 30 years Mr Rossi travelled Europe buying art and furniture and learning more about his passion.
"In the Sixties everyone was neglecting Italian furniture, so he was able to buy it more cheaply," said Mr Tavella. "He was supposed to be buying it to sell but he was like a mouse and he hoarded everything in the five apartments he rented."
The collection became famous, but Mr Rossi was a fiercely private man. "He only let a privileged few, close friends and academics, see it. For the most part it was for his own enjoyment," said Mr Tavella. "It has so many multiple examples of each category of furniture that it can function almost as a guide to 18th-century makers and styles. There is a cabinet which Napoleon presented to Duke Emmerich von Dalberg, and a set of four French 18th-century wall lights from the Louis XV reign. "It really is the most fantastic collection and it is wonderful that it is being sold for a humanitarian cause."Reuse content