Hate that old blouse? Fear not: A slew of new startups are running virtual marketplaces where folks can sell or buy secondhand treasures.

Companies like Poshmark, Twice and Threadflip are offering new twists on the yard sale and what they say is a more intimate experience than online mega-malls like eBay.

"It feels like it's become a new cultural shift, in terms of what women can do with their wardrobes," said Rosalie Yu, a Poshmark user who lives in Dublin, Calif. "It's changed how I shop."

The trend is closely tied to the rise of other "collaborative consumption" startups like RelayRides, Airnbnb and TaskRabbit, which let people easily rent their cars or spare rooms and find help with odd jobs.

"I like the idea of doing something environmentally sustainable that helps people save money," said Noah Ready-Campbell, chief executive of Twice. So when he and a co-worker at Google decided to do their own startup, they saw a way to apply the collaborative concept to their own memories of childhood.

"We grew up wearing a lot of secondhand clothes," explained Ready-Campbell, 24.

His service, launched in March, sends users prepaid shipping labels with which to send in their used designer clothes. (Sorry, gents - the site, like most others in the space, only handles women's items, though that could change in the future.)

After vetting the items to make sure of their condition, Twice staffers make an offer and send cash on the spot. They then photograph the items and curate them into an online catalog.

Ready-Campbell said Twice typically sells clothing for 25 to 35 percent more than it pays for them, a margin he calls similar to high-end thrift shops like Crossroads Trading and Buffalo Exchange. But with extras like two-day shipping and 24/7 customer support, "we basically can create a like-new shopping experience for the buyer," he said.

The business model isn't without risk. Twice, and a similar online marketplace called thredUP that specializes in reselling children's clothes, have to invest in warehousing operations, which can boost costs.

If that approach can be likened to that of Amazon.com, Poshmark's is more like eBay's - a centralized exchange that matches buyers to sellers and takes a cut of the action without ever actually handling the merchandise.

"With our iPhone app, users can take a photo of an item in their closet, like a handbag or dress, and convert that into a listing in less than a minute," said CEO Manish Chandra. If a prospective buyer stumbles across that item in one of Poshmark's forums, the app's mobile messaging feature allows for quick communication between her and the seller. And once the sale is closed on the platform, Poshmark emails the seller a shipping label, then keeps 20 percent of the price.

Chandra and others say this new generation of e-tailing is being driven by the ubiquity of mobile phones, the increasing sophistication of phone cameras and the rise of social networks like Facebook and Pinterest, which let users discover new items by trolling their extended connections.

"We've already got women who are going into their closets and putting together little boutiques, because they know other women love their tastes," said Chandra, who previously sold social shopping site Kaboodle to Hearst Corp. for a reported $30 million.

Yu said she's sold more than 60 fashion items just in the past month using Poshmark; traffic picks up on Friday nights, when women are getting ready to hit the town. She also uses the platform to bargain-hunt for herself, sometimes while waiting in line. "I'm getting 50 percent off luxury goods that are in great condition," she said.