Note: Save the game before trying this trick. Enter Buy mode, and select the most expensive item you can afford. Buy one of it, and place it anywhere desired. Then, select the "Items" tab on the right side of the screen; this will bring you back to the page you bought your item from. The item you previously bought will still be selected. Do not press anything except the "Purchase" button. Even if you cannot afford the item, the game will still allow you to purchase it again. Buy as many of the item as desired. Then, sell all the items you just bought. Repeat this trick as many times as desired. Note: Your money cannot go negative, but this trick will not work as well once you have a lot of money.
The Sims 3