Do you have young kids who like to exchange things with friends?

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I would like to know if any of you parents who have young schooling kids face the problem of having their kids who like to exchange things like pencil, ruler, eraser, etc with their classmates? My daughter, age 7, first year in elementary school, tends to come home with pencils and erasers, that are not her own, in her pencil-case. When I question her as to what happens to her own, she says that she exchanged them with her friend. I am not against sharing of things, which is good, but my concern is whether they know the line between what is voluntary exchange or 'forced exchanged'. I have heard of a case once when a boy, who was eye-ing his classmate's Gameboy, used a small simple toy car to exchange for the item. His classmate, being a quiet boy who happens to like car, 'gives' his Gameboy to the boy. The first boy held on to the Gameboy set for days with no intention of 'exchanging' it back. The quiet boy's grandma told me later that she had to go to the school to ask the first boy for the Gameboy back and return him the car. My concern is whether the first boy will grow up to have the habit of thinking that he can get whatever he wants just by exchaning things for it. What would you do if your kid does such thing? How do we educate them without hurting the part about sharing?