Scott Painter makes his living betting on startup companies, having played a role in launching 29 of them over the years. But with the bad economy choking initial public offerings and acquisitions, Painter is now backing an idea that makes it easier for insiders like him to sell shares in their companies even before they go public.

SharesPost, which was founded by Painter’s business partner, Greg Brogger, launched publicly in June. Through SharesPost's Web site, Painter is trying to sell shares in several companies he helped found, including car pricing startup TrueCar.com. He also wants to buy shares in companies that are far from an IPO, like short-messaging site Twitter and business-networking site LinkedIn. SharesPost is one of a few private stock exchanges that are emerging to fight what venture capitalists call a liquidity crisis. These exchanges give stakeholders an alternative way to trade their shares in hot startups like Facebook for cold, hard cash — without having to wait years for an IPO.

Employees at startup employees often put in long hours but get salaries that can be 20 percent less than their peers at public companies. In return, they get stock or options that they hope will be a path to sports cars and summer homes after their company goes public or is bought out. Given this, services like SharesPost could help startup workers get some cash while awaiting a distant IPO that might never even get off the ground.

And it’s not clear how much — or how little — stock has changed hands through them. In its short life, Santa Monica, California-based SharesPost said it has executed one $25,000 transaction, while another service, New York-based SecondMarket, said it has completed about 40 transactions in the past year worth about $150 million.

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