They say that you cannot time travel when trying to find the best price in the Stalk Market. But that’s not true.
If you and a friend are both playing AC:NL, you should both keep track of turnip sale prices throughout the week (don’t actually buy turnips on Sunday!). If one of you gets a really good price on turnips (say…500+ bells/turnip) the other player travels back to Sunday morning.
Now you have one player in the present (like a Thursday afternoon) when you can sell turnips for a great profit, and another player in the past on Sunday morning where turnips can be purchased from Joan. Now if the person in the present travels to the other player’s town, he time travels to the past and can buy turnips, when the player goes home, he time travels again and none of the turnips spoil.
Both players can get in on this bell-grabbing action.
Rinse and repeat.