The ability to buy and sell stock is called liquidity. In startups and many private companies, it is often hard to sell stock until the company is sold or goes public, so there is little or no liquidity for shareholders until those events occur. Thus, sales and IPOs are called both exits and liquidity events. Sales, dissolutions, and bankruptcy are all called liquidations.
Often people wish they could sell stock in a private company, because they would prefer having the cash. This is only possible occasionally. We get into the details later, in our section on selling private stock.