The paintings, sculptures and sketches from Beyeler and his wife and partner Hildy's private gallery collection include works by Monet, Gauguin, Renoir, Toulouse-Lautrec, Picasso, Matisse, Kokoschka, Klee, Leger, Dubuffet and Roy Lichtenstein, auctioneers Christie's said on Sunday.
"Many generations of specialists and collectors have seen their taste forged by Beyeler's eye," said Jussi Pylkkaenen, president of Christie's Europe, Middle East and Russia.
"To buy from Ernst Beyeler was to buy great 20th century modernism, and to buy from Beyeler was to buy the best," he added.
During his lifetime, Beyeler had already donated much of his collection, more than 200 seminal works, to a foundation and its purpose-built museum next to his native city of Basel.
However, he still owned other cherished works through his small gallery in the city centre.
The Swiss gallery announced late Friday that it was closing in keeping with the last wishes of the couple, who had no children, and that its resources would be used to raise money for the Fondation Beyeler.
"We look forward to a tremendous atmosphere in the saleroom and to raising a significant sum for the Fondation Beyeler, which remains the great legacy of Ernst Beyeler's personal generosity and vision," said Pylkkaenen.
The auction is to take place in London on June 21-22.
The auctioneers described some of the works on sale as "small jewels" that Beyeler could never conceive of selling during his lifetime.
They include a mobile sculpture by Alexander Calder that sat by his desk and a Paul Klee watercolour, "Parklandschaft" (Park Landscape), that graced his bedroom wall.
Another highlight will be one of Monet's series of paintings of Nympheas (water lilies), that have sold for up to $80 million before, as well as less celebrated items such as 30 ceramic works by Picasso.
An economist and salesman by training, the young Beyeler nurtured his passion for art during the 1940s in an antiquarian book and print shop owned by a German exile in the northern Swiss city.
Beyeler took over the business at Baeumleingasse 9 after the owner's death in 1945 and gradually turned it into an art gallery, partnered by Hildy.
He often said he was guided by little more than his own intuition rather than by fashion or trends.
After a first exhibition of Japanese woodcuts in 1947, some 16,000 works of art passed through Beyeler's hands over half a century.
Beyeler's friendships with famous painters were such that Pablo Picasso allowed him to pick 26 of his works during a visit to the ebullient artist's studio at Mougins in southern France in 1966.
He also became a dealer of choice for the world's leading collectors, selling masterpieces to museums, according to Christie's.