The home, which made its market debut at $8 million in April 2006, in the midst of massive renovation intended to set buyers’ hearts aflutter, has lingered there ever since. A handful of renters have come and gone, but none have wanted to sign the deed. Not for $8.5 million (the highest ask), not for $6.45 million (the lowest and most recent ask) and not for anything in between. It’s now listed for rent at $25,000 a month.

When The Observer visited 129 West 20th Street on a recent afternoon, we found an apartment that was many of the things it has claimed to be over the years: “glamorous, dramatic and refined,” just as the first Corcoran listing had promised, as well as “cinematic in scale and scope” like the Prudential Douglas Elliman listing bragged a few years later. (It had, in fact, starred alongside Keira Knightly and Eva Mendes in Last Night and Mariah Carey in an AT&T commercial.)

We couldn’t argue with the first Brown Harris Stevens listing that pronounced it “modern yet ultra-chic, exceptionally spacious and yet welcoming,” or with the second, which declared the condo “a singular home with dramatic potential.”

But after six years of showings and rentals, the celebrated design was starting to show its age: the floors were scuffed, the dust bunnies were reproducing like, well, bunnies, and the handle of a glass door to the terrace came off in our hand. A broker there to scope out the apartment before showing it to her clients—a family with three small children relocating from Canada—wondered why no one had bothered to paint, repair a warped piece of kitchen cabinetry, or sweep.

The owners were probably exhausted by their ongoing stewardship of a property that they’d expected to offload years ago, a place where they’d never lived and upon which they continued to spend $9,000 a month in maintenance and taxes.

Properties between $4 million and $9 million that sold in the third quarter of 2012 spent an average of 161 days on the market, or about five and a half months, according to Streeteasy data, to the Chelsea penthouse’s six and a half years. Was it simply asking too much, or was there something more? How else to explain a highly desirable space that no one seemed to desire, in a city where real estate is an obsession, vacancy rates hover near zero and battles over square footage are fought by the inch?

The penthouse of 129 West 20th Street numbers among a handful of Manhattan luxury properties that aren’t just slow-sellers, they are no-sellers. Not just for sale, but forever for sale.

Take, for example, the ten-room spread on the fourth-floor of the Dakota—one of the most legendary co-ops in New York (where else can one mingle at an annual October potluck with both Lauren Bacall and Yoko Ono?)—that has been on the market since October 2006.

The $14.5 million apartment lacks park views, and its décor is a bit theatrical, but it has 4,500 square feet, six fireplaces, and a shaving closet. Still, after so many years, even the Warburg listing sounds fatigued, sapped of the energy to string clauses and descriptions into proper sentences; it ends with a semi-coherent fade out: “One of a kind. Function and glamour.............As fabulous as your fantasy.”

A few dozen blocks north, on Central Park West, the turreted, red-brick condo conversion at 455 CPW has had a terrible time trying to lure buyers. The former nursing home and New York Cancer Hospital has undergone an impressive makeover, but No. LM17 and LM19 both hit the market in December 2006, where they remain to this day, asking $5.6 million and $3.8 million respectively. Manhattanville isn’t the most prestigious address, and buyers may have an aversion to moving into a building where most residents once left in body bags. But can that really explain a half-decade of snubbing?

And what of the penthouse at 425 East 63rd Street, listed since February 2007? Doesn’t anyone want a $5.6 million penthouse on Lexington and 63rd Street, even if it does look like something out of a Jackie Collins novel? The long-suffering broker, Debra Forest of AIB Management Corp., admitted there were a few things that might not appeal to potential buyers: the penthouse is actually two separate units that would need to be combined, and the décor is “circa the 1980s.” But she remains confident someone will be thrilled to snap it up. “I’m definitely not worried,” she said. “The owner is not worried.”

There might be any number of deficiencies or problems with apartments that linger, according to Jonathan Miller of appraisal firm Miller Samuel, but the only real problem is price. “There’s always a price to match a buyer,” he said. “Even when someone is murdered in an apartment, there’s a price it will sell for.”

What’s more, overpricing can backfire. A study Mr. Miller did with the Furman Center at NYU found that the closer the seller lists a property to its actual market value, the higher the price it actually sells for. And conversely, he said, “if you throw a property on for an absurdly high price, you end up getting less than you would have if you had listed it close to market value.”

Vanity and ego are the two major driving factors behind many outrageous asking prices. Take, for example, the townhouse at 22 East 71st Street, purchased by Aby Rosen for $15.6 million in 2004. After completing a partial renovation, the real estate tycoon relisted it for a whopping $75 million in 2008, making it the most expensive townhouse listing at the time, (that honor now belongs to the $90 million Woolworth Mansion). Mr. Rosen, who never lived in the home, eventually dropped the ask to $50 million in 2011 and the property is now said to be in contract to the Qatari prime minister for $47 million.

“Aby is Aby,” said one high-end broker. “It’s ego.”

Nobody who has had their property on the market for more than three years really wants to sell, claimed Fred Peters, the president of Warburg Realty. Some sellers are parading a trophy, some are testing the market and some are simply delusional. But, said Mr. Peters, “at some point, the pain becomes too much. It all depends on where the threshold is.”

At this point, the seller frequently moves on to a second broker (indeed, most lingering listings leave a string of jilted brokers in their wake), for whom he or she is often willing to lower the price. The Chelsea penthouse, for example, burned through nine different brokers and four different brokerages. And that’s not even counting the rental brokers.

Other reasons that places sit: a fussy co-op board kills a low offer, or it’s an estate, notoriously difficult to sell because the apartments often need renovation and bickering heirs are reluctant to lower prices. But most brokers blame dust-covered listings on unrealistic sellers and the brokers who are only too happy to tell them what they wanted to hear in exchange for a wildly expensive exclusive listing and the publicity that comes with it.

One broker angrily told us that he’s missed out on more than a few listings by being honest with sellers rather than peddling a fantasy number. Brokers who do that “screw up the market,” he complained. “They agree to overprice a property because it’s what the sellers want to hear.”

Moreover, once a listing becomes stale, buyers start to think that there must be something wrong with it. Then, even if and when the seller drops the price, buyers can be leery.

“It’s like seeing a piece of jewelry displayed at a store,” said A. Laurance Kaiser IV of high-end brokerage Key-Ventures Realty. “If it’s been in the window for six years, unless it’s the most exquisite thing that no one can afford, you think there’s got to be something wrong with it.”

More expensive properties do take longer to sell because the pool of potential buyers is much, much smaller for trophy penthouses and sprawling $40 million townhouses. A property between $10 million and $20 million spent on average 225 days on the market this quarter; for properties over $20 million, that number falls to 212 days, according to Streeteasy data. But that’s still shy of a full year.

Not all sellers are greedy, though—some of them are just so infatuated with their homes that they can’t see them objectively. “Psychologically, the problem is that everyone loves their own house the most,” said Kirk Henckels, the executive vice president at Stribling.

Expensive, custom renovations are particularly tricky. One broker spoke of an owner who’d poured millions into a meticulous renovation and felt like he absolutely had to get the money back when he decided to sell. The architect and designer encouraged him to stand firm: after all, what did it say about their design if it didn’t bring in a fawning public?

What happens to these market dinosaurs? Does anyone ever come along to rescue them from listing purgatory, the endless parade of critical buyers judging their views, frowning into their bathroom mirrors and peering into their cabinets?

For a few, five or six years down the road, the market finally catches up to an asking price. The townhouse at 41 East 70th Street, for example, spent seven years on the market before it sold this fall for $25 million—the price it tried, and failed, to get in 2005 before reaching ever higher—$30 million, then $35 million.

Perhaps the buyer, steel magnate Leroy Schecter, felt a pang of sympathy for the overreaching townhouse. He had, after all, just re-listed his apartments at 15 Central Park West for $95 million—a full $40 million more than he’d been asking the year before.

So what’s responsible for the spinsterhood of 129 West 20th Street?

“It’s a luxury loft in a non-doorman building—there’s your start. Nothing in the building is that size, and it’s a very, very taste-specific,” said a broker who was familiar with the property. “I brought a lot of people there who really, really loved it, but it didn’t fit their lifestyle. And they didn’t want to pay for a renovation that if they bought it, they’d have to destroy.”

He cited the cantilevered steel staircase—which would make most parents of small children convulse in fear, the kitchen with unusual zebrawood cabinetry (stunning) and unusually small European appliances (not so stunning) and the fact that the master bedroom and its terrace faced an office building. It was a temperamental beauty whom everyone wanted to date but no one wanted to marry. Not to mention that the Chelsea of 2006 was not the High Line, Google-hosting, gentrification juggernaut it is today. If it made its market debut today, it could probably get $6.45 million, just as it could have back in 2006, but it’s no longer the fresh-faced debutante it had once been.

“The owner really spent a lot of money in the materials and the design,” said another broker. “But in real estate, your dream penthouse is not someone else’s dream penthouse.”

Indeed, although the staircase didn’t worry the broker representing the Canadian family (the children already lived with one) she fretted aloud that the layout—with the bedrooms right off the entertaining areas—would keep the children up if the parents had parties.

Eran Cohen, a rental broker in the building, was more blunt. “I think it’s a great, great unit, but it won’t go for that price,” he said, adding that the owner keeps toggling between renting and selling, which may suggest indecision. Said Mr. Cohen: “He has no patience.”

kvelsey@observer.com