“This will not be a third exit, not in the near term,” said Roger Ehrenberg, a former investment banker and hedge fund chief executive who now runs IA Capital Partners, his angel investment fund. “Just like we’ve seen a very limited or nonexistent I.P.O. market, it stands to reason that these buyers would be the same kinds of investors, so I’m not at all convinced that the demand side is even there.”

For one thing, it could be a thin market, on both sides of the trade. The Securities and Exchange Commission allows the trading of unregistered securities, but only for select investors. The new market will be open only to accredited investors and qualified institutional buyers, which the S.E.C. defines as financially sophisticated enough to invest in high-risk securities about which there is little public information. These include individuals whose net worth exceeds $1 million and institutions that manage at least $100 million in securities.

Even if individual shareholders and venture capitalists do trade on the exchange, it will be hard for it to grow, Mr. Ehrenberg said. There is only a small universe of such wealthy, risk-tolerant investors, he said, and companies are limited to 500 shareholders before they have to file with the S.E.C. as if they were public.

“For this market to really develop real liquidity, that rule needs to be changed, but right now, the government needs to do the exact opposite,” Mr. Ehrenberg said. “They don’t want more people to buy illiquid, unregistered investments  they want maximum transparency.”

As in the public markets, investors will determine how much a company is worth, but with much less information than they have for public companies. SecondMarket will gather public data for buyers and sellers, and companies can choose to submit detailed information that buyers can see. But it is unlikely that many private companies will reveal much data.

Although the securities are unregistered, fraud may not be a big worry. SecondMarket says it verifies with the company and its lawyers before any shares change hands.

“There is no risk that investors wire money and don’t own the stock or the company doesn’t exist,” Mr. Silbert said. The public nature of the market may also serve to discourage swindlers.