No boom lasts forever, and this spring may be remembered as the moment when the art-auction frenzy of the late 1980s began its decline. In the big sales in New York City over the past two weeks, despite freakish prices for two great paintings, the auction market was showing ominous signs of instability. For Van Gogh and Renoir, in Japan, there was no ceiling. For other artists, including some highly promoted contemporary ones, the floor was shaky.
On Tuesday night at Christie's, Van Gogh's melancholy portrait of his physician, Dr. Gachet, sold to the Japanese dealer Hideto Kobayashi for $82.5 million,...