Shadow Market

An unregulated private market in which investors can purchase shares in companies that are not currently publicly traded. Shadow markets in stocks give investors an opportunity to invest in companies prior to their initial public offerings (IPO). However, the SEC requires investors to have a net worth greater than $1 million in order to participate in this nontransparent market. These people are what the SEC refers to as "accredited investors".

Taobiz explains Shadow Market

The main benefit of using the shadow market to purchase shares is that the accredited investor can get exposure to certain companies much earlier than most other investors. This greatly increases the potential profit for the investor if the stock goes public and demand from average investors drives the stock price up. Some of the downsides of the shadow market include lack of liquidity, lack of disclosure from the company, and a greater degree of uncertainty and risk.