Trade is beneficial to both parties under very general conditions (or else it would not happen). This is very often used as an argument in favour of trade liberalisation etc.

However – and because of this fault this is not really an economical argument – this does not take into account the opportunity costs. Say, if a school boy works as a shoe shine he is better off than if he does not work; but only insofar as this does not interfere (negatively) with his schooling, playing piano or football etc.