A situation that was discussed on another site: A goes to a store, sees an item he wants to buy, and puts it into his shopping trolley. It's the only item of that kind left in the store. B comes along. B would also want to buy the item. But the only item left is the one in A's shipping trolley.
So B tries to take the item from A's shopping trolley. A hasn't paid for the item yet, and we assume that B is willing to pay the store. What is the legal situation here? What is A allowed to do to prevent B from taking it, including in the case that B is physically stronger and A is not strong enough to hold on to the item?