If you want to remove an item, either from the Park Editor mode or from your normal park, it will be placed in the Storage Box.

You can retrieve an item from the box at any time, while in your normal park or in Park Editor mode. 
- In the Build Menu, stored items will display the amount currently in the box, rather than their price.
- In the Park Editor mode, the Box Menu will only show stored items, not all cards. It is impossible to buy new items in the Park Editor mode. 

To place a stored item, simply drag it into the park.  Replacing a stored item is free, since you already purchased it.