It is not positive sum, in competitive market it is basically exchanging services, and each party has to agree on mutual value of services being exchanged otherwise no trade.
Accountant values his labor as 100/hr and plumber values his services as 100/hr and they simply exchanche services.
Because the price paid is somewhat less than the value derived. You might have been willing to pay him $110 and he might have been willing to walk away with $90. But regardless of whether you settle on a rate of $90 or $110 or split the difference at $100, a $20 positive sum is created.
You mention competitive markets, where surplus is reduced or almost eliminated. But most markets do not reach such a late stage level of optimization.
