Steam now has its own auction house. Valve is launching what's called the Steam Community Market, a new platform that will allow you to buy and sell items for Steam Wallet cash within various games on the giant digital network.
The service is launching in Team Fortress 2, with more games to come later. For now, you can only buy/sell consumable items:
As part of our ongoing efforts to gradually phase out all Earth money in favor of Team Fortress hats, we're kicking off the TF2-only beta of our new Steam Community Market! The Market should improve trading in every way: People looking for specific items will be able to locate them faster, folks looking to sell items will find the process a lot more efficient, and best of all, we've made it easier for everybody to translate playing TF2 into buying games on Steam.
While the Market is in beta, we're limiting trading, buying and selling to one-time consumable items while we're ironing out the kinks. Also, items you buy will be subject to transaction fees; read more about it in the FAQ.
Valve takes a 15% fee on every item sold, and game publishers will be able to take their own fees as well. There are all sorts of other restrictions—including a 200-transaction limit per calendar year—so you should check out the big FAQ before buying or selling anything on this new marketplace.