Animal Crossing: New Leaf has a Stalk Market, which is similar to the stock market. On Sunday mornings you can buy turnips from a boar named Joan. Starting Monday, you can sell them at Re-Tail, but the sell price changes throughout the week. The changes in price may follow a pattern that allows you to predict when you can make the best profit, but there is always a risk of losing money. However, if the prices are good, you can make a lot of bells easily through the stalk market. This page explains the stalk market in detail.
Play the game between 6AM and noon on Sunday and look for Joan, a brown boar, wandering around your town. Talk to her to purchase turnips from her. Her price for the turnips will be between 90 and 110 bells. Her price doesn't change during the day. The turnips that you buy from her will spoil on 6AM the following Sunday.
Turnips can be stored in your storage, which you can access from the lockers in the train station, or from drawers and closets in your house. If you are in someone else's town, you can access your own storage using the lockers in the other player's station. This makes it easy to sell turnips in someone else's town. But don't forget that turnips spoil at 6AM on Sunday.
If you play the game, then save and quit, and turn the 3DS clock back and play the game with the clock showing the same time it showed when you played before, the game considers this to be time travel. Doing this will cause all of your turnips to spoil right away.
You can avoid all time travel problems by making sure that the clock doesn't say the same date and time as it did when you were playing earlier. If you need to turn your 3DS clock back because of Daylight Saving Time, be sure to wait until the repeated time has passed. For example, if you change your 3DS clock from 1:59AM to 1:00AM, be sure to wait until your clock says 2AM or later before you start playing again.
To find out how much turnips are selling for in your town, go to Re-Tail and ask Reese for the current turnip selling price. The price changes when the Re-Tail shop opens for the day and changes again at noon, so be sure to check the price twice during the day to see both prices.
To sell the turnips, just sell them to Reese the way you sell other things to her. She will only buy them from you after the shop opens on Monday and before the shop closes after Saturday. You have to sell the turnips before 6AM on Sunday or they will spoil and you won't be able to sell them.
There are four patterns that turnip prices can follow throughout the week. They are: Decreasing, Big Spike, Small Spike, and Random. The following sections explain each pattern.
In the decreasing price pattern, the prices will consistently decrease by a few bells each time, never increasing. With the decreasing pattern, the start price for the week will be between 99 and 50 bells. In this pattern, if the price doesn't increase by Thursday afternoon, you should sell immediately.
The big spike pattern features a decreasing pattern, but then there will be three increasing prices, with the third being the maximum for the week, followed by two decreasing prices that are still higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by two increasing prices and always followed by two decreasing prices, so the maximum can't occur on Monday or Saturday.
The small spike pattern features a decreasing pattern, but then there will be four increasing prices, with the fourth being the maximum for the week, followed by a decreasing price that is higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by three increasing prices and followed by one decreasing price, so the earliest that the maximum can occur is Tuesday afternoon and the latest is Saturday morning.
The random price pattern features unpredictable prices in the range of about 50 to 200. The prices increase and decrease at random. The price seems to go over 110 at least twice a week in the random pattern.
Keep in mind that with the stalk market, there is always a risk of losing bells because you might get the Decreasing pattern, which will almost always be too low to give you a profit. The other patterns typically feature a price over 110 bells at least twice during the week, so if you track the stalk market each day and see any increasing price, you should be able to make a profit even if you miss a spike.
The best way to reduce the stalk market risk is to make friends with other players who follow the stalk market in their towns. Even if your own town has the decreasing pattern this week, there is a chance that one of your friends' towns will have a better pattern. You can even mark a friend as a Best Friend, and if your 3DSs are both connected to the Internet, you can send each other messages about the current stalk market prices in your towns without having to visit.
To choose the best time to sell, you have to check the prices whenever they change, and use that information to determine what pattern you have. This section explains how.
For the best stalk market results, be sure to check the prices every time they change. They will change each time Re-Tail opens for the day, and again at noon. If you forget to check, it becomes much more difficult to accurately detect which pattern you have.
By checking the prices each time, you can watch for an increase. If the price increases, it means you don't have the Descending pattern, so you can watch for a spike.
If you have seen an increasing price, wait for the next price change. If it decreased right after an increase, it means you have the random pattern, so you should sell if the price goes over 110 bells. However, if the price increases two times in a row, you should wait and check the next price.
If the third increased price is 250 or greater, you should probably sell right away, because it's probably the large spike pattern. If the third increased price is not that high, you should wait until the next price change and sell then, because it's probably the small spike pattern.
If the turnip prices haven't increased by Thursday afternoon, you should sell your turnips right away. The latest that an increase can happen is Thursday afternoon, so if there haven't been any increasing prices by then, it means you have the decreasing pattern and will need to sell at a loss.