Exploitativeness is not necessarily ill-defined. It's just that when you define it rigourously, it either never happens or happens too often for the definers' ideological persuasion.

And by the way, your statement is also extremely ill-defined. If you say that most transactions are not zero-sum, you are presupposing some kind of value system, which you didn't specify, and the value system for which that is true are also very ill-defined.

It's also true that a positive sum interaction could absolutely be exploitative. So the actual amount of exploitative interactions is strictly higher than the number of zero sum or negative sum interactions.