You need to use your lifestyle points to buy them from the back of the house menu that pops up when you click on an empty lot. You need to have at least 20 lifestyle points to buy a themed house but the game should give you enough anyways.
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No you can buy a prebuilt house or move in some houses from the inventory thing in edit town.
the mail box is always in front of a house or a lot when you buy one.
They are in your Family Inventory.
An empty lot means that you have to build the house yourself.You can buy a house, though.You have to choose one. Sorry!
No you cannot buy a gun on The Sims 3.