Mr. Gross had reported on his blog that Courtney Sale Ross’ duplex at 740 Park, which Mr. Cave has been unofficially marketing for over $60 million, was off the market. “Who’s Michael Gross?” the broker said in an interview posted to The Observer’s Web site this Monday. “If you call Edward Lee Cave, and you are qualified, you can see the apartment.”

No one quite knows the story of uptown’s huge but hushed listings—all well over $45 million. What are the spaces actually like? Which sprawl has the best chance of selling? What kind of would-be buyers have actually been in them, and how many? “Less than 20 people, more than 10,” Mr. Cave said Tuesday of the 740 duplex. “Twenty percent are old money; 75 percent are people that have been successful, very successful, in the space of the last 10 years.”

His client and her late husband, Time Warner creator Steve Ross, made the duplex by adding a 14-room sprawl to an 18-room sprawl, which, according to two top brokers who have seen the apartment, is problematic. “If you’re combining a bunch of different apartments, what do you think is going to happen to the flow?” one said. “Two good apartments put together,” said the other, “does not one great apartment make.” (This reporter visited the duplex once back in boyhood and got lost.)

“That apartment is appropriately designed for the lifestyle the Ross family had,” Mr. Cave explained. “The apartment is quite flexible to be reworked for other requirements.”

When you call up Manhattan’s top brokers to ask about a quiet listing, the main thing you’ll learn is that each has its own problem. “People love the idea of living at 834, but it’s twice what it’s worth,” said a broker who’s brought a client to billionaire Les Wexner’s unofficial $60 million listing at 834 Fifth Avenue. “It’s not a hugely high floor; it has no outdoor space.”

“It’s plain vanilla; it’s not tricked out,” said a broker who’s shown the Time Warner Center’s north tower penthouse, for which Nu Skin cosmetics company co-founder Sandie Tillotson wants $80 million.

“Anyone is going to rework that listing, meaning physically, architecturally,” said Leighton Candler, who has brought two out-of-town clients there. “Both felt that it was very attractive, but it wasn’t lavishly enough finished. It is really not lavishly finished. The finishes are not the finishes of an $80 million apartment.”

Nearby, apartments at 15 Central Park West have been said to be asking as much as $90 million, even though they were bought for less than half that last year. “I’m impressed with the building at its original prices, but there’s nothing to me that merits those prices,” a top broker said. “I don’t find it compelling at all. Not even to go see.”

But last week something compelling happened: the billionaire J. Christopher Flowers, who paid a record $53 million for the Harkness Mansion on East 75th Street, was reported to be quietly asking $49.95 million for the townhouse, though broker Sami Hassoumi has not listed the place officially.

“There’s someone who marked below what he paid—one can argue that he shouldn’t have paid what he did, though I’m not making that argument,” Ms. Candler said. On the plus side, Mr. Flowers’ plans to renovate the now-gutted house have already been approved by the city. “If he’s really providing those plans, selling for less than he bought it for—and it’s likely the biggest house on the Upper East Side on the market—he’s delivering a value.”

mabelson@observer.com