Because of the small volume, size of the companies listed, and the nature of private equity, there is no public exchange to buy and sell shares on.
The goal of most companies is to get big enough to list and IPO on a public exchange, allowing free transacting of their shares.
Regulations restrict the re-sale of Reg CF investments for 12 months after the purchase of the security, unless it is to an accredited investor, and investments made under Reg A+ are fully tradable upon closing.
Sometimes the issuer will buyback shares, some already have the option in the investment contracts, sometimes it takes years to have an exit opportunity, and many companies never have an exit.
In the near future, Newchip plans to launch a secondary marketplace bulletin board trading system to facilitate the sales of shares to other investors.