In early 2005, Richard Polsky decided to put his much-loved, hard-won Warhol Fright Wig, up for auction at Christie's. The market for contemporary art was robust and he was hoping to turn a profit. His instinct seemed to be on target: his picture sold for $375,000. But if only Polsky had waited . . . Over the next two years, prices soared to unimaginable heights with multimillion-dollar deals that became the norm and not the exception. Buyers and...