Sotheby’s has guaranteed the collection of its one-time owner, Alfred Taubman, for “approximately” $500m, prompting Sotheby’s to increase its credit facility temporarily from $600m to $800m. More than 500 works, from antiquities to contemporary art, will be offered in a series of four sales beginning this autumn in New York. The estimated value makes it the most valuable collection ever to come to auction. Taubman, who died earlier this year, was a shopping mall tycoon who bought Sotheby’s in 1983 for $125m. His reputation was marred after a conviction by a federal jury in 2001 of colluding to fix art prices with Anthony Tennant, his counterpart at rival firm Christie’s. Taubman was fined $7.5m and sentenced to ten months in prison. (Tennant was also indicted but refused to travel from the UK to New York for trial and so was never charged.) The first sale from Taubman’s collection, titled Masterworks, is due to be held on 4 November.