THE DEMAND for art today is higher than ever before. Auction houses report record turnovers, and absolute top prices are being paid in increasingly rapid sequence. Only three years ago, Turner’s “Seascape: Folkestone” had an auction price of $9.8 million at Sotheby’s (London). A year later van Gogh’s “Landscape with Rising Sun” was sold for $9.9 million at Sotheby’s (New York), and Mantegna’s “Adoration of the Magi” sold at Christie’s (London) for $10.4 million. Enormous publicity greeted the sale in April 1987 of van Gogh’s “Sunflowers,” which was sold at Christie’s (London) to the Japanese insurance company Yasuda for $39.9 million.