How does Cookie Clicker's stock market work?

This doesn't really answer the question but I don't think I really need to answer it since the post by aphid already did that way better than I could ever hope to. I just want to say a quick tip about the stock market. I suggest buying a stock or two right at the beginning of the game, doesn't matter which one as long as you keep the stock till you need some extra cash since the amount of cookies the stock is worth increases as your CPS increases. So if your CPS is only 10 can you buy one stock of sugar for 5 dollars/ 50 cookies and when your CPS Rises let's say to a thousand you can sell that one stock of sugar which is worth 5 dollars for 5,000 cookies. Of course, that's ignoring the overhead cost and the possibility of the Stock's value changing but as long as you buy the cheapest stock you should generally be able to sell the cheap stock you bought at the beginning for a little bit of extra change later.

TL;DR Buy some stocks at the beginning to sell later