How do I get into the Steam Trading Beta?

From the Steam Client, open Steam->Settings and select the ‘Steam Trading Beta’ option. The Steam Client will then need to restart in order to download and apply the change.
What items can be traded?

Team Fortress 2 items and Steam Gifts can currently be traded in the Steam Trading Beta.
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. 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.
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?

Once you have opted in to the Steam Trading Beta, you can select Community in the Steam client, then on the right is a link to View Inventory.
Who can see my items?

Your inventory obeys your existing regular privacy settings – public, friends only, friends of friends, or private.
Who can I trade with?

Right now you can initiate a trade with anyone in a Group Chat or on your Friends List who has also opted into the Steam Trading Beta.
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?

We’ve set up a forum thread here where you can post all feedback relating to the beta.
I have found a bug in Trading, how do I report that?

Please report all bugs in the same forum thread for giving feedback. Please click here to go to the thread. We will be actively reading the thread during the beta.
I think I just lost items in a trade! What do I do?

You’ll need to Contact Steam Support and create a new ticket. Please include your account name or a link to your Steam Community profile in the Support Ticket, as well as all the details you can remember from the trade. It will also be helpful to include a list of items that you have lost, a link to the Steam Community profile of the user that you traded with and the date and time of the trade.
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.