I would somewhat agree with you if the marketplace would not be operated by Valve. But it is. This makes very clear that the one of the intended use cases of the skins is to be sold. Which is understandable. It's very likely Valves makes in the order of a billion dollars just from the marketplace.
A person could reasonably argue can be exchanged vs primarily intended as a stand in for cash is important. If the items are intended as actual items people value then that's more defensible than say chips that only exist to be cashed out. (And I'm not familiar enough to know whether that's the case here)