• The penthouse inside Central Park Tower is the most expensive listing in the country.
• It has seven bedrooms, eight bathrooms, three powder rooms and a 2,000-square-foot ballroom.
• For the year, sales of apartments priced at $10 million or more have declined 38%, according to Miller Samuel.

A penthouse atop the world's tallest residential building − listed for $250 million − is marking the biggest test of the ultra-luxury real estate market at a time of falling sales and growing economic uncertainty.

The three-story mega-home inside Central Park Tower, which spans more than 17,500 square feet, is the country's most expensive listing. It is also the highest, situated at over 1,400 feet and spanning the 129th to 131st floors. Perched on Manhattan's "Billionaires' Row" − a strip of super-tall skyscrapers along the southern edge of Central Park − it is being marketed as the ultimate real estate trophy for a billionaire looking to tower over New York City.

"I've been selling real estate for 15 years now, and I've sold some of the most expensive real estate in New York, Florida, everywhere," said Ryan Serhant, of Serhant, who is marketing the penthouse. "I have never seen anything like this apartment."

The big question is whether the listing can fetch its asking price as storm clouds gather over real estate, financial markets and the broader economy. Luxury real estate sales in Manhattan have slowed dramatically in recent months. The number of signed contracts for properties priced at $5 million or more fell by nearly half in August compared to a year ago, according to a report from Miller Samuel and Douglas Elliman.

For the year, sales of apartments priced at $10 million or more have declined 38%, according to Miller Samuel. The most expensive sale of the year in Manhattan so far is a $74 million penthouse of the new Aman New York condo.

Some brokers say the $250 million asking price for Central Park Tower penthouse is unrealistic.

"I consider this a fantasy price," said Donna Olshan, Manhattan the luxury broker.

Olshan said there have been 23 closed sales in the building this year, with an average price-per-square-foot of $5,228. The penthouse, which is much larger with higher ceilings, views and amenities, is seeking more $14,000 per square foot.

But Serhant said the price is appropriate, given the sale of a penthouse at nearby 220 Central Park South for $190 million, or $20,000 per square foot.

"I know it sounds crazy, bur relatively speaking, it's priced at a great value on a per-square-foot basis," he said. "It's just a very, very big apartment with lots of amenities."

The triplex has seven bedrooms, eight bathrooms and three powder rooms. A stairwell that winds its way up through the three stories is the centerpiece of the main salon, and a 2,000-square-foot ballroom on the top floor has 27-foot high ceilings.

Central Park Tower was built by Extell Development, the developer behind several of Manhattan's new super-towers. To protect the privacy of would-be buyers, Extell and Serhant are limiting public viewings of the unfurnished apartment to a few select areas.

The home has the highest terrace in the world, a glass-rimmed platform soaring 1,460 feet above Manhattan. It also comes with a lavish list of building amenities, including a 60-foot outdoor pool, 62-foot indoor saltwater pool, spa, private garden, game room, conference room, fitness center, squash court, screening room, private restaurant with Michelin-star chefs and a wine and cigar lounge.

Serhant said the apartment's greatest amenities are the 360-degree views, with Central Park spreading out below like a green welcome mat and hills of New Jersey and New York suburbs visible in the distance.

He said he has already seen strong interest from the ultra-wealthy, who are less affected by stock-market declines, rising rates and recession fears.

"The purchaser of this apartment is someone who is looking to diversify their assets," he said. "It's someone who probably owns expensive art, probably has an expensive car collection and other things, and they want the best of the best."

Serhant said one billionaire was flying in this week just to see the apartment.

"When they saw it come onto the market a few days ago, they reached out and said 'Is this the best apartment in the world?' I said 'yes,' and they said 'I'll fly in to see it'."