1. If you add a New Store in the Store tab on the right:
Choose Sells None Items in the List or Sells All Items in the List:
2. If you add a New Store in item's Details:
Choose Sells This Item Only or Sells All Items in the List:
If you already assigned Any item to a Specific store, the app remembers that, and next time you add the item to you List - it will automatically assign the item to that store.
In order to unlink an item from a store - take off the check mark from that Store in item's Details->Store list
If you have too many "wrong" items assigned to a Store - might be easier to delete the Store and re-add it considering choices mentioned above.