As a result, developers have had to be more strategic to stay competitive. “You need to build in the right neighborhood for the right buyers, or offer something totally unique and in great demand,” said Kenneth S. Horn, the founder of Alchemy Properties. He said sales at 250 West 81st Street, one of several new projects, have gone especially well because “it was a new condo and there are not many new condos in that area.”

Individual sellers, however, were slower to adapt to the changing marketplace, and saw their properties linger. As of the fourth quarter, it took an average of 152 days for a listing in Manhattan to go into contract, up from 101 days the same period in 2017, according to Garrett Derderian, the director of data and reporting for Stribling & Associates.

Over all, the average Manhattan apartment price slid nearly 5 percent from 2017, to $2.06 million from $2.16 million, according to a year-end market report by CityRealty. (The drop would have been steeper without the pricey new-development sales.) Closed transactions for all condos and co-ops were projected to total 10,354, the report said, with sales reaching $21.3 billion. That’s down from the 13,295 transactions and $25.7 billion in sales in 2017.

“The larger story is volume,” Mr. Levy said. “While it’s down, it’s not falling off a cliff.”

New Developments

Several luxury condominiums filled up in 2018, though the pace of sales and the average closing price declined significantly from the year before.

“Purchasers have more opportunities to select among many properties — they know they have choices, so they’re taking their time,” said Susan M. de França, the chief executive of Douglas Elliman Marketing.

Developers also had to contend with fewer international buyers, who in past years were active in the new-construction sector. Some foreign buyers now face tighter restrictions for moving money out of their home countries, particularly in China.

To help facilitate sales, developers were more willing to negotiate prices. “When the inventory level was tight, you would never hear the word ‘negotiation’ in the development market,” Ms. de França said.