Qualified Small Business Stock (QSBS) has special tax rules which can result in no taxes when sold. To qualify, a business needs to be an active C Corporation, less than $50MM in assets, not in an excluded category, acquired directly (not second-hand), and the stock must be held for 5 years or more and have documentation of the stock. The excluded (not taxed) gain is limited up to $10MM or 10 times the adjusted basis of the investment (after that it gets the normal capital gains treatment).