The IPO market has all but disappeared, and lofty merger and acquisition deals are unthinkable. So entrepreneurs, as well as their employees and investors, are left with few opportunities for unloading their stock.

SecondMarket, a New York City financial services company, thinks it can help. In April, SecondMarket launched Private Company Marketplace, a site on which businesses set the terms for sale of their privately held shares.

Barry Silbert, CEO of SecondMarket, says he got the idea last year after brokering the sale of more than $50 million in Facebook shares and receiving a slew of requests for similar sales at start-ups such as Twitter, Tesla Motors, and Glam Media.

The service taps SecondMarket's database of 3,000 investors to match buyers, mostly institutional investors, and sellers, mostly employees. Both buyer and seller pay a 2 percent transaction fee.

There are risks when a company permits employees to sell. For one, they may be less motivated to stick around. "It's a good idea conceptually," says Aaron Patzer, CEO of Mint, a Silicon Valley Web company that has raised $17 million in VC funding. "But you don't want frustrated employees selling options."