The Flea Market is an event in Animal Crossing: Wild World and City Folk that occurs every first Saturday of every month excluding January and August, and continues all day long. During this day, you're able to go to all of your neighbors' houses and ask to buy some of their items, where they'll either offer a price or politely decline, stating that they simply want the item.

Also, if you wait in your house, some of the animals will enter, asking if you'd sell some of your items, where you'll then be able to set a price, or, as they did, decline for your favorite items.

In Animal Crossing: New Leaf, the event is removed but, Re-Tail takes over most of the function in that players can put up stuff to sell and occassionally villagers will put up their own stuff to sell if there is open space.