They'd be losing the same as they do now with everyone that uses 3rd party sites to sell instead of their market.
They'd still get market fees. Most people would still be selling their skins as they get tired of them, and there would be people on the lookout to save a fee $€.
As soon as they removed something from their ingame shop, prices would rise and their cut would go higher.
All that changes is that you would pay directly for the item you want instead of buy a key to possible get nothing from it.