Does each free and voluntary trading (buying/selling) transaction result in a winner and a loser at the end of the negotiations over price?

Of course not.

Each side feels like they are better off with the deal than without. Otherwise, why would they participate? So, by definition, it’s a win for both sides.

But suppose one side has a monopoly and can jack the price up?

Same thing. If the buyer participates, it is because the buyer expects to be better off with the deal than without. Even at the higher price.

Besides, monopolies don’t last long in a free market. When other participants see the price the monopolist is getting for his goods, there will be a mad scramble to bring more of that product, or viable alternatives, to the market to get in on the profits. And, this will quickly kill off the monopoly and drive the price down.