You can make a huge profit by buying white turnips from Joan and then selling them when Tom Nook will buy them for more than you paid.
What is the Stalk Market?
Tom Nook runs a stalk market, where he will have a random price that he will pay you for any white turnips that you might have.
When can I sell white turnips to Tom Nook?
Tom Nook's stalk market is open every day except for Sunday, because that's when Joan sells white turnips. Tom Nook's turnip prices change at noon each day.
What is the point of the Stalk Market?
Joan the turnip seller will sell you white turnips. Her selling price will be different each time, but it's usually around 100 bells. The point of the stalk market is that Tom Nook's buying prices are random, and sometimes he will buy turnips for a much larger amount than you paid for them. If you have a bunch of white turnips, you could make a lot of money!
When does Tom Nook have the highest prices?
This is pretty much random. To try to get the best price of the week, you have to check in with Tom Nook every day, before noon and after, to find out what his turnip price is, and then you just have to guess whether the price that he tells you is going to be the best price of the week or not. Just be sure to sell the turnips when his price is higher than the price you paid for them, and you will profit.
If you make a note of Nook's prices every day, you might end up finding a pattern to the peak prices that can help you predict the trends.
When do white turnips spoil?
White turnips only last for a week. They spoil at 6AM on the Sunday after you purchase the white turnips. However, there is a glitch that lets you keep white turnips for as long as you want....
How do I do the table trick for white turnips?
I consider this to be cheating, but if you go into your house, stand in front of an empty table or other piece of furniture that you can put items on top of, and put some items on the floor around the table that you can walk on top of, then try to drop the white turnips and they will end up on the table (as long as you have covered up all possible places on the floor where they could go). When they are on the table, they will never spoil. If you put them into your pockets again, just be sure to put them back onto the table before 6AM on Sunday to make sure that they won't spoil.
Why are Tom Nook's prices so low?
If you use the telephone in your attic to change the time in the game, you will get penalized in various ways, one of which being that Tom Nook's stalk market prices will be really bad. You might even discover that Joan will not sell you any white turnips on Sunday after you change the game time with the attic phone. You can avoid these penalties by only changing the game time by changing the DS clock time before you start the game, and not using the telephone to change the game time.