You know that überstylish friend who’s your exact size and who always lets you raid her closet? Yeah, we don’t have one of those, either. But the new startup Poshmark, based in Menlo Park, makes buying (and selling) previously worn clothing nearly that simple, without the scam potential of eBay or the drag of shlepping things to consignment stores. “Every three years, a trillion dollars of clothing gets bought,” says cofounder Tracy Sun. “And until now, there was no easy exit for it.” Just download the app, log in with Facebook, snap a photo, enter the details (size, description, original price, and your asking price), and post. (You can even tweak your photo with an in-app Instagram-like photo editor.) Poshmark holds the buyer’s money until she gets the item, then pays the seller through her PayPal account, via direct transfer, or even with an old-fashioned check. The company does take a 20 percent cut of any sale, compared with eBay’s 9 percent, but the process is so stunningly simple for sellers—Poshmark sends you a prepaid shipping label, so all you have to do is pack up the item and drop it off—that eBay would do well to be worried.