Entrepreneurs have one common denominator - they don't take no for an answer...

There's nothing quite as satisfying as proving naysayers wrong.

For many entrepreneurs, starting a business means hearing friends, family and experts say the idea won't work. The timing is off. The market is terrible. The idea isn't practical.

Sometimes, though, entrepreneurs know something that the critics don't. It's more than just a gut instinct. They grasp some fundamental aspect of the situation that others don't—a nuance of the market, for instance, or what makes potential customers tick. And sometimes that insight pays off big-time.

Consider what happened to Ann Patchett. A few years ago, the best-selling author thought about opening a bookstore in downtown Nashville, Tenn., where she lives, but people weren't kind to the idea.

They said "opening a bookstore was the stupidest thing you could do," says Ms. Patchett, who's now 49 years old. "You might as well be selling eight-track tapes. It's dead; it's over."

But Ms. Patchett thought there was room for her business. Why? There had once been two bookstores in the area, a Borders and part of a smaller chain.

"Both were more than 30,000 square feet and profitable" but were closed because of big changes at the chains, she says. So, she reasoned, if the city had been able to support 60,000 square feet of profitable bookstore space, "why can't I open 2,500 square feet of bookstore?"

Now Parnassus Books is thriving. Ms. Patchett's partner in the venture, Karen Hayes, says it has exceeded their sales projections every month they've been open.

Listen to the Public

In many cases, naysayers are thinking too broadly, says Victor Cheng, an author and business coach. Most critics focus "way too much" on economic conditions, he says, and not enough on whether the business can meet unsatisfied customer demand.

"If someone finds that customer and figures out a way to solve their headache in a profitable way, that company's going to succeed regardless of economic or industry conditions," Mr. Cheng says.

So, he says, "entrepreneurs should ignore all the friends and family critics and only pay attention to one voice—the voice of potential customers."

Genevieve Thiers followed that logic. Twelve years ago, as a college student, she saw a woman posting fliers to find a baby sitter. Ms. Thiers had what she thought was a great idea: a website that would link mothers with baby sitters in their area.

But when she took the idea to investors, they didn't think much of it. "A lot of people said moms will always turn to their friends" for baby sitting, she says. Others were downright insulting: "One said, 'My wife handles that' and another told me, 'It's a baby sitters club.' One guy just started laughing, so I left."

She agreed that parents often turned to friends as baby sitters. But she figured "parents were increasingly transient, so when you move to a new area, as a mom, you have no network."

The mothers she consulted supported the idea. "It was a weird dichotomy, to have one half say no and the other half say yes, but it never occurred to me to listen to the investors," says Ms. Thiers, now 35. "Perhaps if I'd had an M.B.A. I would have, but for me, it was as simple as talking to the customer."

To recruit sitters, Ms. Thiers distributed fliers at dorms in Boston, where she then lived. She hired friends to build a website for her company. "It was very bootstrapped," she says, adding that she "literally had to borrow $120 from my dad to buy the domain name."

Today, she says, Sittercity Inc. is an international company with millions of users world-wide.

Frosting on the Cake

Likewise, Natalie Shortridge didn't get discouraged by critics when she decided to open a bakery in Abingdon, Va., in 2009, featuring just cupcakes.

When Ms. Shortridge—a stay-at-home mom at the time—told people about the idea for Babycakes Cupcakery, they came back with a couple of big objections. They told her she'd never make it offering just one item. And it was no time to open a business when so many were closing and people were cutting out luxuries.

But Ms. Shortridge reasoned that in tough economic times "people cut things like traveling and buying new cars and houses and still treat themselves to great food."

Besides, she figured, people like to pick up cupcakes for birthdays and other events, and "cupcakes are the perfect size—portion control—so you don't have half of a cake left over to either eat or throw away."

Ms. Shortridge, now 34, also liked the fact that her concept didn't require a large initial investment. She prepared cupcakes in the kitchen of her home and sold them at a local farmers' market. Later, she rented kitchen space in a coffee shop.

Now, even in the teeth of the recession, "business isn't slowing down and we are not having to live on a loan as a lot of businesses do for the first several years," Ms. Shortridge says.

Things have been so busy, in fact, that she has opened a second location and moved the original store to a new spot. The new building also came equipped for a full restaurant, so "we decided to go for it" and add a food menu, she says.

Mr. Green is a writer in Chicago.