what consitute a shoplifting in a store?

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Ok, I had posted a discussion about people being wrongly accused of shoplifting while they were still doing the shopping at the store. Now my question is what really constitute a shoplifting and it is not? What is the thin line out there to measure the difference? What are the criteria and store rules to accuse a customers shoplifting on the premises? What if that particular customer wasn't shoplifting at all, and he/she is willing to pay those items? What are the consequences?