In Manhattan, some sellers of luxury real estate have slashed prices. The asking price for a Park Avenue townhouse dropped $18.5 million, to just under $30 million; a seller cut the price for a Central Park South apartment by $7 million late last year, to below $18 million, and has since taken off another $2 million. Neither property has sold.

The billionaire hedge fund manager William A. Ackman, who was part of an investor group that paid $91.5 million for an apartment at One57, the tower on what is now known as Billionaire’s Row in Manhattan, may already have missed the market peak if he hopes to flip it at a profit. An apartment there purchased last April for $20.3 million sold again this year at a $2.5 million loss.

Such prices are still in the unattainable stratosphere for almost everyone but the ultrawealthy. Still, the impact is significant: 12 percent fewer Manhattan apartments (189) were sold last year for more than $10 million than in 2014, with most of the decline coming in the second half of the year, according to CityRealty, which tracks co-op and condo sales.

In London, the trend is even more pronounced. Prices for central London luxury properties over all dropped in 2015, with the steepest declines in two of the city’s wealthiest neighborhoods, Belgravia and Knightsbridge. The number of high-end sales dropped 40 percent in December, according to Property Vision, an English property advisory concern. It recently reported that sellers were cutting prices, and “we are negotiating deals today that would have been impossible even in the late autumn.”

Art collectors are anxiously waiting for the results of the big spring auctions in New York. The art market’s first test of the year, the February auctions in London, were not the disaster some had feared, but results were tepid, with total sales declining 35 percent at Christie’s and 50 percent at Sotheby’s.