I'll answer your question with a question of my own. Trade summary reports are produced for each stock, each trading day. In those reports, 'buy' volume is reported separately from 'sell' volume, and those numbers almost never total to the same amount. Since every trade has a buyer and a seller, can you think of any reason why everyone in the industry would go to the trouble of reporting that info, and why those buy and sell totals would differ? If you are wondering why I have not simply answered your question, just refer to the proverb:
Give a man a fish and you feed him for a day; teach him to fish, and you feed him for a lifetime.
I'm confused here. If there is a purchase isn't there a sell?