The Vanderbilt family is selling a J.M.W. Turner masterpiece that was considered “one of the finest watercolour-drawings in the world.” “Bamborough Castle” — which was believed by many in the art historical world to be lost — has been in the family’s collection since 1890, when Cornelius Vanderbilt purchased it. But at a time when art prices, especially prices for Turners, are soaring, the family has decided to sell.