Answer: A defendant can comit the crime of shoplifting without actually leaving the store. All he need to is to move the property and exercise control over it in a way that is inconsistent with the shop owner's reasonable expectations as to how shoppers will handle merchandise.
For example, shopkeepers know that people will handle an item to examine it, purchase it, try it on, show it to a companion, and so on. Shoppers may even move the item to another department, to compare it with other merchandise. The critical question is how that movement and carrying is accomplished. For instance, a woman who places several blouses over her arm, takes them to her husband in another part of the store, and shows them to him, probably does not have the intent to steal them. But if she takes those items and places them in her backpack, or under her coat, a proseuctor may be able to convince the jury that these actions indicate an intent to leave without paying for them. Accordingly, guards or salespeople may approach and detain someone who has taken these steps, even before the person leaves the store.
Many clerks and security personnel will not, however, apprehend a suspected thief until that person has actually left the store. The reason is not that they lack justification before that moment, as just explained. Instead, they simply want an open-and-shut case. It will be difficult to argue that one intended to pay for the goods when one has walked out without doing so; on the other hand, defendants who are apprehended pre-exit may be able to convince the jury that their actions were consistent with an intent to eventually pay for the merchandise before leaving the store.