Valve are now letting you buy and sell items using real money and the Steam wallet. The new Community Market is currently in beta and only available for Team Fortress 2 but will soon be expanding to other games.
It's quite the exciting thing.
People have been making money out of hat sales in Team Fortress 2 since the game went free-to-play, but money could only be made by people who were making hats and selling them in the store. Valve are now bringing all transactions in-house.
They're calling it the Community Market and what that really entails is that any eligible item you find in Team Fortress 2 can be put up for sale from your inventory. Then other players can buy this with funds from their Steam Wallet - which can only be spent within Steam.
So, for instance, if you're playing Team Fortress, happen upon a crate with a new hat for your sniper, one that doesn't appeal, you can stick it up for sale. If someone likes it they can pay you for it, adding to your wallet's balance. Getting you that little bit closer to a copy of Super Hexagon.
As this beta expands out to other games the potential market becomes quite exciting. Portal map makers could start earning a little for their work or Skyrim modders could begin to make a living (assuming they eat digital games to survive). It's a beta experiment at the moment and you can read up more about it at the Community Market group.
One thing worth noting, looking at the FAQ, is that "All purchases are final – you will not have any right to a refund or a reversal of any Community Market transaction once it is completed."