Valve has added a new feature to Steam’s trading system – Trade Offers, allowing you to suggest what you want in return for the goodies you’re giving away.
As detailed in an announce post, Trade Offers are restricted to users on your Steam friends list, and can be initiated from either party’s Inventory or badge pages.
In a Trade Offer, you select both what you have to give, and what you’d like in return. The other party is notified, and can accept, decline, or counter-offer. You can send multiple offers with the same item, but when one offer is accepted, the rest will expire. Offers will automatically expire after two weeks, and can be manually cancelled
The major advantages of the new system are that trades can be initiated while one party is offline; trades can be sent and received in-browser rather than through the Steam client; and you don’t have to negotiate via chat with a human being (good for people who order pizza online).
If you’re keen to have a go, be sure to set your inventory to public so your friends can see your goodies.
Valve may open the system to all users, not just those who are already friends, in the future.
Steam trading cards are earned in random drops triggered by playing games, and as such are a kind of achievement system. Some players offer their cards for sale on the Steam Marketplace, and there is quite serious demand from some quarters to obtain full sets, presumably for the bragging rights.