Once a week, a turnip seller will come through town. This works just like the real world "stock market". What happens is she offers you the turnips at a price - say 100 bells each. You can buy up a bunch of them. They only take up one inventory slot, no matter how many you buy. Now every day you go to Tom Nook's store and he has a special prompt to tell you what the current turnip price is. One day it might be 90 bells. Another day it might be 200 bells. It's up to you to figure out when to sell. On one hand you don't want to sell "low" and lose money. On the other hand, the turnips go bad after a week so if you never sell, you have lost all the money you spent on those turnips.

It's a lot of fun, and can make you a lot of money if you're diligent about checking his prices every day!

Animal Crossing Wild World Walkthrough