Why would people in a voluntary exchange choose to continue a trading agreement that isn't beneficial?

[Falkland G]

A very polite member of the forum, whom I nonetheless strongly disagree with, chose my answer to one of his questions as Best Answer but then, as a comment, posted this “Well I think unrestrained free trade is like Social Darwinism. Stronger beings tend to devour weaker beings in nature. It's not unlike what Germany did to Poland.” My question is this: “In what way can two consenting adults who are in a state of voluntary trade “devour” one another? When Germany invaded Poland, Poland was not consenting to this. When the National Socialists threw people into gas chambers the victims did not consent to that either.
John Nelson : Trade is always a double inequality equation. If I buy a newspaper for two dollars this means I value the newspaper more than the two dollars. It also means that the newspaper seller values the two dollars more than the newspaper. To the extent there is “exploitation” it goes both ways.