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.

[John Nelson]

This goes back to the idea of the greater of two evils. One person simply may not have a choice to stop trading because stopping would be worse. In this way, the strong (the one in control of the resources) is forcing the weak (the one that needs to maintain the thing that is slowing destroying it) to continue. I'm not sure what he meant as far as the Germany/Poland thing. It doesn't really fit.

It isn't that you value the newspaper more than two dollars. The need for the news is what you are valuing. The newspaper is just a piece of paper with patterns of ink on it. Since the newspaper vendor controls the resources, he doesn't need the news. And if he can't sell to you, he'll sell to someone else. Thus he has power over you. Someone IS going to buy his paper. But if you want it, you need to submit to his conditions. When it comes to food and medicine on a macroeconomic scale, this need is intense. So intense that the seller can value it at whatever price they wish.

[Vendapin]

“When it comes to food and medicine on a macroeconomic scale, this need is intense. So intense that the seller can value it at whatever price they wish.”

John Nelson is wrong when he said that a seller can value a product at whatever price he or she wishes. For one thing, there is competition – especially when it comes to something like food. So called “food deserts” exist in the United States not because of a lack of nutritious food items – such things are quite available – but because the consumers in these areas are not demanding it.

Even if a "monopoly" were to exist there would still be a price the market could not sustain.

To answer your question, people in a voluntary exchange WOULD NOT choose to continue a trading agreement that isn't beneficial. A convenience store that sold nothing but green leafy vegetables in one of the “food deserts” in the United States would not be in business long because the people there would not buy his products. Likewise, if a restaurant consistently sold food that tasted disgusting the patrons would not return.