Mrs Justice Rose held that Sotheby's were entitled to rely on the connoisseurship and expertise of their specialists who had examined the painting thoroughly and that they reasonably concluded that "the quality of the painting was not sufficiently high" to indicate that it might be by the Old Master.
http://www.standard.co.uk/incoming/article9983154.ece/alternates/w620/cardsharps1.jpg An art lover whose painting was sold by Sotheby’s for £420,000 then later insured for £10 million today lost his High Court bid to sue the auction house for negligence. Lancelot William Thwaites now faces the threat of a legal bill many times higher than the price he had received for the painting. His family had acquired the work known as The Cardsharps in 1962. When he decided to sell on inheriting it Sotheby’s catalogued it as by a “follower” of Caravaggio and it went for a hammer price of 42,000 in December 2006. It was bought by the partner of art scholar Sir Denis Mahon, who, a year later, declared it to be the work of the old master from around 1595 and worth 10 million.