Trigger to show layer based on condition of object in a different slide

Hi, Kimberlee.

Thank you for sharing your screen recording. What a fantastic idea!

There are different ways to approach what you're trying to do. I'll share one suggestion, and I hope other community members will offer other options as well. 

You can use variables and states to check/show objects that were dragged into the inventory in different slides. Instead of adding extra layers for each added object, you can have all items in a hidden state until the user adds them to the inventory. 

Take a look at my example below, with three rectangles, all hidden initially:

I created three True/False variables (default value = false), one for each rectangle:

When the user opens the inventory layer, Storyline checks if the variable is True, and if it is, it changes the state of the object to normal:

On the base layer, when the object is dragged to the inventory, the variable is set to True:

Here's a preview, and I'm also attaching the .story file so you can check my example: