You shouldn't take advantage of people.
Sean plays a card game and is trading with James, who knows very little about the game.
Sean wants one of James' valuable cards.
Sean helps James realize the value and offers a rare card for the one he wants.
Sean offers James a trade using several common cards for the rare card.
James accepts the trade and is happy with it.
James accepts the trade, realizes later that he was cheated, and loses respect for Sean.