Economies are zero sum. Your statement is a fallacy I see touted everywhere.

All economic products cost energy to generate and all energy is limited in totality by the available energy on earth.

When someone uses this energy, another person cannot use it.

This holds true even for the most abstract product. Say the economic product of poetry produced/sung by a poet.

The poet needs energy to sustain himself and the time needed to produce the poem. When he uses that energy, entropy increases and someone else now cannot use it.

That is zero sum.

The transaction you are describing is the equivalent of two people taking gold from limited reserves and trading that gold. The fact of the matter is, the gold is still missing from the shared reserve regardless of how fair the trade was.