Steam – How does game trade work

Per the Steam article on the subject

Extra copies are not given in most cases.

Only titles purchased in special promotions as outlined below are eligible for giftable extra copies

There has also been some discussion in gaming chat about the ethics of giving away extra humble bundle keys, as the site says the keys are for your sole use.

The original discussion was here. Ullallulloo emailed Humble Bundle support and asked for us. He posted the reply in chat here. Here's what it said:

"Our Steam keys do not generate giftable copies on Steam. Please consider your bundle a "unit" and not to share copies of the games or Steam keys from it.

If you'd like to give a copy of the games away, we have a system in place that allows you to buy a gift key from www.humblebundle.com so your friends can have access to direct downloads as well as any Steam or other keys we have included.

Please let me know if there's anything else I can help you with.

Thank you for your support"

For non-Humble Bundle trading, I would suggest the meta post related to game trading.

Steam software is listed under the "All Software" category in the Steam client, not the "All Games" category.

To view it, use the dropdown menu located just to the right of the search box, and select "All Software"