Compare and contrast the law of demand vs. the law of supply. Keeping the law of demand and the law of supply in mind, consider the following: on a typical day, there are millions of stocks sold on the stock market. The number of shares sold always equals the number of shares purchased. This means the quantity of each company's shares demanded equals the quantity supplied. If that is true, why do stock prices constantly change? Doesn't this go against the law of supply and demand?