You can trade unopened gifted copies, extra copies. Just open up Steam, click on Community, On your profile, click on View Inventory in the actions menu, on the right. Whenever there's a good deal, I'll sometimes opt to get a 4-pack when it's cheaper than buying one at regular price, since it gives me an opportunity to give it to a friend. That, sometimes, leaves me with extra copies.
Here's their trading FAQ:
Spoiler!
What items can be traded?
Currently, Team Fortress 2 items, Spiral Knights items and gifts purchased on Steam can all be traded between one another.
Will there be other games that support trading?
We’re working now with other game developers to incorporate both Trading and Inventories. Portal 2 should be reasonably soon and we hope to have several third-party games in the next few months.
What do you mean by trading Steam Gifts?
Any game you’ve purchased from the store as a gift, or received as an Extra Copy, can be traded to other users. They can be used to trade for other Gifts, or for items in Team Fortress 2 or Spiral Knights. We’ve added a new checkout option to the Store when purchasing a gift so you can save it for trading or sending later, to support users who want to save games for trade fodder.
What restrictions are there on trading Steam Gifts?
Trading a Steam Gift is very similar to sending a gift to someone; just that once it is traded that user can then either trade it again, or open it for themselves. There are no restrictions on territory.
Why aren’t some of my gifts tradable?
Only gifts that are purchased on trusted accounts can be traded immediately; otherwise there is a 90-day cool down before the gift can be traded. Any account that has made any valid purchase from the Steam Store more than 90 days ago is considered trusted. This is to ensure that all games traded are valuable, and help prevent fraudulent purchases for the purpose of trades.
Does Steam Trading mean I can sell my used games?
No, only games that have been bought as a gift, and thus have never been played, can be traded. Once the Steam Gift is opened and added to your game library, you won’t be able to trade it again.
How do I see my Inventory on the Steam Community?
You can select Community in the Steam client, then on the right is a link to View Inventory.
Who can see my items?
Your Steam Gift Inventory is now only visible to you while we work on new privacy settings. The rest of your inventory obeys your existing regular privacy settings – public, friends only, friends of friends, or private.
Who can I trade with?
You can initiate a trade with anyone in a Group Chat or on your Friends List.
How is this different from trading in Team Fortress 2?
We’ve improved the display of changing which items are in the trade, and significantly increased the number of items that can be traded in one session. We hope these improvements will help with confusion or users trying to deceive others in the trade.
I have ideas for other things in Steam that can be traded. How do I give that feedback?
You can post feedback or suggestions in our Suggestions/Ideas forum here as it is regularly read by our developers.
I think I just lost items in a trade! What do I do?
Please read our Steam Trading Policy here for further information.
What’s this new Friends Tagging feature?
It’s not related to trading, but in this new Steam Client is also a new feature to help you organize your friends list. You can right-click on any friend and select ‘Tag As…’ to choose a category for them. Friends can be tagged in multiple ways, and will show up then several times in your friends list, once for each tag.
How do I add a game to my library after receiving it in a trade?
Go to the game in your Steam Inventory and click “Unpack gift…” to add it to your library.
What if a game I received in a trade is fraudulently purchased?
If this happens, the account where the fraud took place will be suspended. Your account will not be suspended, however the game you received will be removed from your account. We will do our best to restore the items/gifts that were associated with the trade back to you. Please contact Steam Support if a game that you traded for is missing from your inventory or account.
How do I purchase a game and place it in my inventory?
Purchase the game you want as a gift. Select the option "Store the gift in my inventory to send later". Once the purchase is complete you will be able to view the game in your inventory by clicking on the Community tab and then click on 'View Inventory' under the right hand column titled 'Actions'. Select the Steam tab and this will open the games/gifts that are currently in your Steam inventory.
How do you handle scams?
Scams are investigated and action is taken care of the scammer that is reported. We do not provide any information about the action taken against the scammer. To avoid scams make sure to not agree to any of the following:
- Trading items/gifts for money
- Trading items/gifts for CD keys
- Trading items/gifts and expecting them to be returned to you through a verbal agreement
- Trading items/gifts for nothing in return in the first trade and expecting to get an item or gift in a later trade