The is a difference between the trade being zero sum, and the negotiation being zero sum. In the apple example, the trade is not zero sum, but the negotiation is. If I negotiate to spend less money for the same amount of apples, then the seller has lost as much as I gained. The caveat being that even in this simplified apple example there are multiple dimensions to negotiate on. I could buy more apples at a lower cost per apple, which may very well be benefit both parties.
