Earlier quoted context omitted.
Trade is generally not zero-sum, but trade implies transaction between relative-peers. Nation states. Corporations. Groups that have similar amounts of power (Or that can lean on friends.) Many transactions that happen in a capitalist society are very much not transactions between peers. And those can absolutely be, and often are zero-sum, or negative-sum.
Can you give examples where transactions result in negative sum due to asymmetrical power, and what the proposed solution is? From my limited perspective, unless you actually do get rid of private ownership imbalances will exist and Power imbalances will exist regardless of the economic system.
Start by reading literally anything in https://en.wikipedia.org/wiki/History_of_labor_law_in_the_Un... and observe that most of the implemented solutions largely consisted of curtailing the rights of capital holders to do whatever they please in the employer-employee relationship. Weekends, sick leave, overtime pay, company scrip, employer responsibility for worker safety, fair employment, the right to free association, the right to work without harassment... Capitalism didn't produce any of those things - it is incapable of producing them - but has fought tooth and nail, sometimes with batons and bullets against them.
Power imbalances continue to exist, the purpose of all these work-arounds to them is mitigating the harm that someone with power can inflict on someone without.