There are
not always losers, though, in a very real sense. If I have nothing but 100 gallons of water and you have nothing but 100 pounds of meat, then an exchange of some amount at the margins benefits both of us in every way. If we set the amounts that we trade carefully enough, then we'll both win by the exact same amount (however we measure that in our own heads), and it's really hard to call anyone in that situation a "loser".
Presumably what you're referring to is that in a realistic market, there are also tons of other people who are willing to swap their wares, and maybe they'll offer me something better for a gallon of water than a pound of meat. So sure, I could be taking a sub-optimal deal by trading with you at the rate you want. But the trade is still positive-sum, no loser in sight. In an efficient market most trade is net beneficial to everyone and doesn't necessarily have losers, including when you start layering in credit and derivatives and inflation and velocity of money and whatnot, even if it gets really complicated. That is possible because we all have slightly different self-valuations of money, goods, and risk at different scales, and the markets let you trade each for the other until you've self-optimized your portfolio blend.
That's not to say you can't come up with scenarios where there are unambiguous losers, but it's definitely not some law of economics or anything like that.