First go into buy mode, and purchase the most expensive items you can afford. Place it, then go back to buy mode, and don't touch anything other than the purchase icon. The item will still be selected, and you will be able to purchase it as many times as you want without your money going anywhere! So place the item all over your house, and then go back to Grab mode and hit the Sledgehammer, and sell all those items... your money will keep on going up, even though you technically didn't pay for the items in the first place... I know, great glitch, right?