I'm not sure if this a good method, but it seems to be working:
Create an object (sprite for example), set Global=Yes, add NoSave behavior to it.
Add instance variables that will store the items you bought. When player buys something in the shop, put it in these instance variables.
In "On Load Complete" event read these variables and add the items to player's inventory.
Or I guess you can do this with Local Storage. When exit from the shop, write all inventory into the local storage.
In "On Load Complete" load inventory from the local storage.