Since all exchanges in a market economy are voluntary, how do we know that the exchanges will probably benefit all participants

It depends if what you have in exchange is a commodity or product that other party wants and is in need of.  If they are satisfied with what they have received during the exchange and you are satisfied with what you received in return then that means that the exchange was beneficial to both parties.

the correct answer is individualism and exchange. at this stage, children begin to question the rules, because they can see that there is more than one side to them. different individuals have different viewpoints. they won't be able to reach this stage because of ignorance, a lack of ethics or personal biases. that is because people with that characteristiscs are not able to distinguish what is good or what is bad and they can't pursue their individual interests.

should congress have the power to interpret the constitution? write a paragraph in which you argue for or against the wide application of the limplied power