Yeah. We joke about Steam itself being a game, especially during the sales, but it's crazy how much economy-oriented game design must go into the development of the platform. It's in a lot of ways like F2P games... except in this case, weirdly, it's gamified spending. "Free to buy"?
Exactly. For example, in that tumblr post they say that they're given an exclusive item that's worth $25 on the community marketplace. They're only seeing the transaction from a single side - they're selling the item to convert it into Steam wallet credit that can only be used on Steam which thus gives Valve more money.
The part their missing is that, once sold, the item isn't "used" (unless it's a one-time use item like a TF2 key). That same $25 "exclusive" item can be resold an infinite number of times, and each time it's sold Valve keeps a percentage of that transaction.
As long as Valve can maintain that item's rarity / marketability, they can continue to collect residual income on the transfer of ownership of that item indefinitely. By cutting off the supply at a certain point (like, the item was a pre-order bonus that is no longer available) they can actually cause the value of said items to increase as they change hands between collectors and market speculators.
Now multiply this by the thousands upon thousands of items in the Steam marketplace, and you can see why it's kind of a big deal.
