NEW YORK—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 the SharesPost Web site, Painter is trying to sell shares in several companies he helped found, including car pricing startup TrueCar.c...

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