I understand that once a game is in the library it cannot be removed, only hidden. My question is, can I gift an uninstalled copy of a game to a friend? I understand that this may be impossible due to legal issues, and that once a game is in the library it stays.
From the information on the Steam Support Knowledge Base under Steam Gifts they have this question:
Can I gift games I no longer play to my friends?
You can not gift games that were previously purchased on your Steam account to friends.
Even though the question is about previously played games, the answer seems to indicate any games that were purchased for your account.
At this time it's not possible to transfer games from your account to another, It is however something that's routinely discussed on the Steam Forums; Giving up ownership and transferring it to someone else as a gift that is.
Currently the only way to gift a game is to buy another copy, Only spares can be gifted at this time.
The other users already stated it's not possible.
There may be two other possibilites, but I don't think it'll fit your requirements:
- Steam Family Sharing: Currently in beta (only the "host" has to download the Steam beta) you can share your library to other PCs. They can access and download games from your library and also play them. Currently playing a shared library is only possible when its owner is NOT playing a Steam game a once.
- Wait. There have been statements that Steam wants to create a little market place for trading second-hand. But... user cried for that for years and there is no date set. Maybe it won't happen anyways. Try 1. :)