Well, in this case you can save the state of each item in a set of global variables or in a dictionary. When player picks up an item, set variable (for example, change Item5Unlocked from 0 to 1), or add a key to the dictionary.
The Inventory layout should have its own event sheet. Add all item sprites to it and set them invisible. On Start of layout check each variable (or dictionary key) and set the corresponding item sprite visible. For example, if Item5Unlocked=1, set Item5Sprite visible.