Um, are you intentionally being dense?
Of course valve created the marketplace. The skin is gambled, no money changes hands, good deal. The skin is then SOLD on the marketplace, for "Steam bucks" (which as far as valve are concerned are real money, since they exhange them for real money themselves), and valve takes a cut of these steam bucks (again, real money to valve, because they are used in exchange for goods and services on a market that they control), and then the person who gets the money uses them to buy keys on the marketplace, or directly from valve (Again, they get a cut of this, because it's once again real money to valve), and THEN finally that person sells those keys on third party sites at a slight loss (This is the only step of the process valve doesn't directly profit, IN REAL MONEY, from).
It's not the same thing unless Chuck E Cheese's starts running a token-based craps game between patrons in their back alley, and takes a cut of everyone trying to cash out from that. I have no doubt that there are some shady ass Chuck. E Cheese's around, but I somehow doubt that's a standard corporately sponsored policy in any case.
The part that you're describing is analagous to the part where they sell you a key to open the box. The part that that doesn't cover is the part where then the item you get from the box has a cash value that they then facilitate the gambling with and sale of, and continue to rake profit in every time money changes hands.
