And when one of the parties is a group of men with guns who abuse their neighbors in order to produce the something they're selling to the other party, it becomes exploitation in a quick hurry.
I use any number of professionals’ knowledge or skills or supplies just the same as I use natural gas to heat the home or water to hydrate myself or clean whatever.
Maybe something about the seller (or buyer) being under duress would be a start to defining exploitation.
Two parties agreeing on a price for something is not exploitation. Both parties benefit from working together.