You will notice that you also have a Storage option in a few of the game's shop sections, which is where you can place any items you do not immediately use for later purposes. Your Garage can only hold a limited amount of items and cars, so using Storage is vital for keeping things tidy.
If you have previously purchased something that you now no longer need, you can drag and drop it in the Recycling Box. This will earn you Coins and clear up space for other cars and items. Note that you will lose the items permanently, and can not necessarily get them back!