When actor Brian Kerwin decided to sell his longtime Manhattan home—an 1880s Romanesque townhouse he and his late wife had carefully restored—he was hoping it would go for about $12 million, based on similar sales in the neighborhood. When he and his agent, Kim Mogul Wright of Douglas Elliman Real Estate, agreed on a price tag of $8.5 million, he thought there would be “five takers within a week,” said Mr. Kerwin, 69, who has appeared in movies such as “The Help” and “27 Dresses.”

So he was shocked when the Upper West Side...