Steam – CSGO Weapon Case Key Problems

You can't give away a Steam game license, they are account-bound. You can, however, use Steam Family Sharing to let other people play your games when you aren't playing (or when either of you is in offline mode, wink-wink nudge-nudge). Family Sharing is currently in closed beta, so you'll need to wait to get access or for it to become public for an undetermined period of time, though.

Under most consumer protection law codes around the world, there is the concept of "Fitness for a purpose". You would have bought the game disk expecting it to be fit for the purpose of being installed and run on your machine. If the game requires a Steam code for it to be run successfully, then it should have been supplied.

Since the Steam code was not supplied, the item you purchased is not fit for its expected purpose.

Since it is the Steam code that allows you to install (and download) a working copy of the game, this transaction could be considered fraudulent - obtaining financial advantage by deception - as the media you purchased does not include the license to use it, and indeed the seller could legally make and sell as many copies a he wanted (for the cost of the media plus postage) without violating copyright, as long as it was made clear that the transaction is for the media and does not include the license to use the software on it. Not mentioning this latter fact makes this transaction fraudulent.