[dragonwriter]

> Every voluntary exchange is a net plus or it wouldn't occur.

This generalization is justified if, and only if, there are no externalized costs and the perfect knowledge of resulting utilities pillar of the rational choice model accurately reflects reality; neither of these are true generally of real-world transactions, so the generalization is unjustified.

Like the world without friction of many introductory mechanics problems, the world of Econ 101, while it may be a useful foundation for building an understanding of the real world, is very much not the real world.