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What do fashion mogul Rachel Zoe and Hollywood heartthrob Ashton Kutcher have in common?

They're both investors in Poshmark, a website and app that offers women a new way to sell their used clothing. Essentially, they can use Poshmark to turn their unwanted clothes into cash.

"Poshmark is a really interesting sort of virtual closet where women can either swap or buy each other's clothes. That's what's making it really compelling," Jane Buckingham, CEO of consumer trend forecasting company Trendera, told ABC News. "It's different than the other sites or apps like it because it's sort of got an Instagram feel, it feels very user-to-user. It feels like you're inside someone's closet."

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Women can offer dresses, jeans, tops, shoes, bags and accessories up for sale and have them sold in just minutes. They must take a photo of each item, describe it, post it for sale and wait for offers come in.

Manish Chandra, the CEO of Poshmark, explained the genesis of the concept.

"Poshmark really got born out of our own closet," he said. "My wife has bags and bags of fashion that were lying there unopened. And that gave me the idea."

The site has more than 300,000 closets that are active. For a while, one of the sellers included actress Vanessa Hudgens, who used the social marketplace to sell her clothes for charity.

Poshmark has been a lifesaver for Hannah Ouimet, a 21-year-old college student from Chatham, N.J.

Asked how she first got started on Poshmark, Ouimet said: "I got a sweater that looked terrible on me so I decided to try it out and it sold in 20 minutes."

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Now, she uses the site to run her own jewelry business. She has more than 2 million followers.

Ouimet, a fashion merchandising major at LIM College in Manhattan, estimates that she's made more than $80,000 in sales since she started on the site in 2012.

"It helps a ton with expenses," she said of how her business on the site has helped her.