The prewar duplex had five bedrooms, two fireplaces and herringbone-patterned floors. But the co-op apartment at 730 Park Avenue was, nonetheless, a flop with buyers.

There were no takers at $6.75 million in 2017, when Corcoran first listed it. There were still none at $5.95 million, in 2018, when Sotheby’s got the listing. Or, at $5.5 million. Or, at $4.95 million. In fact, the magic number would be $4 million, which is what the apartment sold for in mid-January after being marketed for three years.

“Welcome to the new normal,” said Nikki Field, the Sotheby’s agent behind the deal. While the Upper East Side apartment was hindered by a ground-floor location and dated interiors, a bigger challenge was pricing, she said. “We’ve been behind the market and chasing it downward.”

As the city’s real estate market continues to struggle, with values seeming to erode within the course of months, figuring out how much to charge for apartments has become increasingly difficult, according to sellers, brokers and appraisers.