Valve always got a share of that. None of the items can be duplicated, so steam was properly payed for each of them.
Let's say you buy a csgo key from bitskins so you can sell it for wallet funds... Valve earned money from both the original buyer who purchased it from the csgo ingame store and from the community market player to player sale.
Most of these sites are still out there. Valve would make much less money if they took them down.