Exchange is a Purposeful Action
1) All purposeful action involves a preference of one state over another.
2) Exchange is a purposeful action.
3) Therefore, exchange involves a preference of one state over another.
Premise #1: The state preferred is that which occurs with the successful action. The state being rejected is that which would have occurred had the action not been taken.
Premise #2: Exchange is purposeful, it has a goal in mind: to receive something by giving up something else.
Conclusion: This necessarily follows from #1 and #2: because exchange is a purposeful action, it involves a preference of one state over another. The state preferred is that with what is gained and lost in the exchange. The state being rejected is that where what is gained and lost in the exchange is not, respectively, gained and lost. The more common way of saying this is that he who engages in an exchange values what he is receiving over what he is giving up.