Waiting around for your favorite tech start-up to go public is so last decade. Investors eager for shares of still-private firms have found another way in through the murky but fast-growing secondary markets.

While Goldman Sachs's (GS) Facebook coup is grabbing all the headlines, wealthy investors have been snapping up shares of dozens of other tech firms like Twitter, LinkedIn and eHarmony in online marketplaces for private-company stock. But individuals thinking about joining in on the action should beware: These secondary markets are rife with risks, from a lack of transparency to an abundance of complexity.

SecondMarket and SharesPost, the two best-known markets, began offering trading of private-company in stock in 2009. So far, growth has been dramatic: SecondMarket says it sold $400 million worth of shares in 2010, up from $100 million a year earlier. SharesPost declined to give dollar figures but says it did about 50 private-company stock transactions in the first six months of 2009 and 150 in the following 12 months.

All this sound a bit familiar? That's because secondary markets had their last big run during height the late 90s dot-com boom, when firms like OffRoad Capital and Corpfin.com held online auctions for shares of Internet start-ups. These marketplaces "died under their own weight" in 2000 when the tech bubble burst, says Jeffrey Sohl, director of the Center for Venture Research at the University of New Hampshire. And now like then valuations for popular firms are getting pretty frothy: Some Facebook investors, for example, are offering shares for as high as $67. That implies a valuation of $151 billion, bigger than Coca-Cola's (KO) $146 billion.

Driving today's renewed interest in private stocks is the lackluster IPO market, especially for tech firms, says Brian Erb, a partner in the corporate department of the law firm Ropes & Gray. As a result, many of today's hottest firms are staying private longer even as they become household names, he said. Sites like SecondMarket and SharesPost give investors a chance to buy shares from early-stage investors or employees.

But buying stock at SecondMarket and SharesPost isn't like placing an order with a typical online brokerage: Sellers can handpick their buyers, often preferring individuals or institutions with previous early-stage investing experience, and buyers should have a lawyer review any contracts. SecondMarket, a broker registered with the Securities and Exchange Commission, says it checks to make sure buyers are accredited investors meaning they have a net worth of at least $1 million or annual income of $200,000 or more before allowing them to participate in auctions. SharesPost isn't a broker, but it matches buyers and sellers, who must negotiate directly with each other to make deals.

The growing popularity of these secondary marketplaces has drawn the attention of regulators. The SEC has contacted SecondMarket with questions about pooled investments in private company stock; a SecondMarket spokesman says the firm is cooperating with the agency. A SharesPost spokeswoman said the firm doesn't comment on confidential discussions but that it's keeping the SEC informed about activity on its site. An SEC spokeswoman declined to comment, but Erb says one of the SEC's chief concerns is that secondary markets are permitting private companies to sell shares like public companies, without providing investors the basic financial information that's required of a publicly-traded company.

Indeed, that lack of disclosure makes investing in private companies one of the riskier bets around. Private companies don't have to make the regular disclosures of audited financial data sales, expenses, profits that are required of public companies. They also don't have to disclose key events like changes in management. That's why angel investors typically meet with a company's management and examine its books before committing to an investment. Those opportunities aren't available to investors buying shares in secondary markets, Sohl says. "You're kind of flying blind," he says.

And without basic financial data, it's nearly impossible to determine how much a company is worth and therefore, how much you should pay for it. Goldman's deal with Facebook, for example, implies a valuation of $50 billion. But on SharesPost, sellers are offering Facebook stock for between $30 and $67 a share, which implies the company is worth between $68 and $151 billion. Analyst reports available to SharesPost members estimate the company is worth $10, $13, or $49 billion. "When you look at the secondary market, the valuations are subjective," says Larry Chaityn, the president of the New York Tri-State chapters of the Keiretsu Forum, a network for angel investors. So subjective, in fact, that there's bound to be an increasing number of disappointed investors, says Erb, and some serious consequences: "I think sellers will be sued," he says. "I don't know if those lawsuits will hold up or not, but they're coming."

Buying private-company stock is also more legally complicated than buying a share of a public company. Investors in private companies negotiate their own individual deals, so buying shares from an early-stage investor on the secondary market amounts to buying a contract designed by that seller. Common restrictions in those contracts include a "right of first refusal," meaning any sale of those shares would require a waiver from the company, or a "drag-along agreement," meaning the holder of those shares agrees to vote in favor of any sale of the company approved by its directors or major investors. Later investors' agreements can also invalidate earlier investors' agreements, Sohl says. It's common for later-stage investors to negotiate a guaranteed return of twice or five times their original investment meaning that if the company is sold, those later investors get their money first, and earlier investors split whatever's left.

Perhaps most importantly, buying shares of a hot private company doesn't guarantee a huge payoff if and when the company goes public. Buying Facebook shares now may be better than leaving money in cash, says Chaityn, but not as lucrative as typical angel investors might expect. Investors who miss that very early stage might be better off waiting until after an IPO, when they can review financial information and see how the market reacts to the stock, he says, and suggests an alternative for investors looking to avoid the hype around hot social media companies: Buy Goldman. The Facebook vehicle is an example of the company's ability to execute smart deals that can generate millions of dollars in fees, Chaityn says.