Bob purchases an item from ACME stores Ltd but it was stolen from him before he left the store. Bob would like to request a replacement or refund. Whose loss is this? Whose property was stolen (presumably by another visitor to the shop), and if Bob’s, is ACME liable due to it taking place on their property?
When Bob buys a thing, it becomes his, and it ceases to be the property of the seller. By "buy", we understand that to mean "pays for and receives physical control of". At that point, Bob is responsible to control of his new property. His ownership of the property is not contingent on him leaving the store.
You might assign blame to the shop if they were negligent in some way, for example if they hire a thief to do the exit-check and the door guard takes Bob's property. Obviously, the thief is ultimately liable, but the store might under special circumstances be liable if they indirectly caused his loss. A store does not have an obligation to guarantee that a customer immediately and securely exits the store after making a purchase, so they are not liable for failing to immediately eject him from the store after buying the goods.
Unless there is a term in the contract where the seller maintains risk in the item until it leaves the premises. Such terms are not uncommon in contracts where a product is to be shipped but generally wouldn’t apply in a brick and mortar establishment.
There is a case on point in australia where a bank was not liable for money stolen from customers during a robbery in which the fly-up screens were activated - anything outside the screens, including a just made withdrawal that the customer had not touched was held to be at the customer’s risk. From memory, the bank may have covered the losses as a PR gesture even though they were not liable.