If you mean that only inventory objects should be visible, maybe something like this:
mySprite->Is boolean instance variable set (isInventory)
Action: mySprite->Set visibility (Visible)
mySprite->(NOT)Is boolean instance variable set (isInventory)
Action: mySprite->Set visibility (Invisible)
If you mean to highlight the Inventory objects (that perhaps you can move around, like in Electric Box 2 (space key) or Chessnut
(Flashlight function) ), that's a bit more complicated, I can show you how I did it if that's what you mean, just let me know, write @Geo in your reply here and I'll post a simple capx.