Save 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.
How to make alot of money without a job
First you go into furniture or build mode and buy what you can afford thats the most money. Then put that into storage. Then keep buying it again and again until you can't buy anymore or until you don't want to.(If you don't have enough room in storage then put them all over your house. Then go into the inventory and sell them all unless you want to keep them. Just to let you know once you sell all your items don't do it again unless you have the right amount of money. (I used the wall flat-screen) I hope this helps you.
You don't have to go to the store
If you have an item at home and it's from the store and it's in the storage buy something first before you take that item out of storage. Put the item in your house. Then go to storage and click on the item from the store and put it in your house. After that go back to the buying section and select buy on the item that is highlighted. Then it will charge you the same amount of money that the item is that you took out of storage.