Was there something unlawful about the customer's possession of the item inside the store?
Lawful possession and ownership are not the same thing.
The customer had lawful possession. Title (ownership) is what had not yet transferred out of the store until the item is paid for.
Was there something unlawful about the customer’s possession of the item inside the store?
Now, with your contorted view of things, answer the question:
When did that item become her personal property? Before she actually made a contract of sale with the retailer by exchanging something of value (money) for the property? Or, when it was unpaid for and taken off the shelf within the physical confines of the store’s facility? If she decided not to pay for it and simply abandoned it in the basket within the store, was it still her personal property?
Come on! Use some common sense.