I don't see how that quotation, or the preceding example Marx uses to illustrate it, backs up your strong claim that Marx thinks "no trade is ever beneficial to

I don't see how that quotation, or the preceding example Marx uses to illustrate it, backs up your strong claim that Marx thinks "no trade is ever beneficial to both sides". He's making an economic argument about a certain notion of value. It's not a nihilistic "woe is everything" argument about how there's no point or no ethical way to do any kind of trade. And at least one aspect of his argument is also uncontroversial in modern economics. Take the metric of GDP, and the example of a neighborhood of a ring of 5 houses, each with a mother and child. If mother 1 pays mother 2 for day care to raise child 1, and so on in a circle, then each mother is paying and being paid the same amount to raise a child (just not their own), and each child is being cared for. There is no economic benefit to this shuffling arrangement (in practice there would be downsides like taxes, but ignore all that), however GDP is increased by 5 times the cost to care for a child. But nothing is profited and nothing is lost, and the system cares for no more/fewer children. So while I'm no expert (I'm not a Marxist and I've barely read any Marx or other political economists in their source works), but from what I know, such a strong claim isn't justified, and it reads like a misunderstanding based on the use of "value", which Marx often though not always makes pains to disentangle from notions of exchange value, use value, sometimes ethical and intellectual value (of ideas, etc.), and probably more besides.