Get Rich Quick
Note: Save the game before trying this. When you enter "Buy-Mode", select the most expensive item you can afford. Buy one of it, place it wherever you want. Then select the items tab on the right hand 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, make sure not press anything except the Purchase button. Even if you can't afford the item the game will still allow you to purchase it. Buy as many as you want and when you're done sell all the items you just bought. Your money can't go negative so everything you sell will just give you more money. You can do this trick as much as you want but it may become more difficult to do the more money you have.