Define how the stock market is not a zero-sum game. Assuming there are no unowned stocks, the only way I can make money is for a stock price to go up. The only way for a stock price to go up is for there to be greater demand. The only way for there to be greater demand is for more money to be thrown at that stock. The only way to provide that money is to add more money from outside (which means that person loses money) or for money to be taken from other stocks by selling them, causing those stock prices to do down, losing money for those stockholders.