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.
Stock options, RSUs, job offers, and taxes—a detailed reference, including hundreds of resources, explained from the ground up and made to be improved over time.