From Team Fortress Wiki
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.
— TF2 Official Blog
The Steam Community Market is a sub-section in Steam Community that allows players to trade in-game items to other players for Steam Wallet money.
Items can be put up for the amount of money the seller wishes to receive, or the amount the seller wishes the buyer will pay. All items put up have fees added to them. A Dota 2/Team Fortress 2 fee of 10% of the amount the seller gets and a Steam transaction fee of 5%, adding up to an added amount of 15%. In order to sell an item on the Steam Community Market, the user must buy one item on Steam per year, and then wait 30 days after purchase to begin buying or selling. In addition to this, the user must have a premium account in Team Fortress 2 to sell Team Fortress 2-related items.
The Market is currently limited to Team Fortress 2 Festive weapons, Botkiller weapons, Genuine and Vintage quality items, Dota 2 consumables and invites, and Counter-Strike: Global Offensive items.