While we do have a pretty cool shoplifting mechanic in Identity, the issue with stealing items being sold by another player is the potential for exploitation. The ability to steal an item is based on its value and the shoplifting experience of the thief, so expensive items will be nearly impossible to steal. A stolen item cannot be sold in a store again, but can be sold in a sort of black market.
So, in short, you can steal from a player owned store out in the world but what you can't steal from are player's private stores, which are usually operated from their own home.