Go to Build mode, then go to wallpaper and press Square to sell. Select a wall that has no wallpaper. Keep selling the wallpaper that does not exist for easy money.

If you need some money and own a "Defective Arcade Genie Lamp" you can keep making wishes and you will get money eventually. However, the genie can also cause fire and make all the characters in the area of the lamp to pee themselves if you make him mad. When the genie is mad he will come out of his lamp with red colored smoke beneath him. However, if he is giving you something good the smoke will be purple or pinkish. It is best to perform this when you first get a house so that you can get your money and not risk losing any items you have purchased. Note: If you talk with the genie, he will also give you small advice and hints occasionally.