Originally Posted by Terms of Use

When you complete Steam’s registration process, you create a Steam account ("Account"). Your Account may also include billing information you provide to Valve for the purchase of Subscriptions. You may not reveal, share or otherwise allow others to use your password or Account except as otherwise specifically authorized by Valve. Any use of your Account with your login and/or password is deemed made by you and you are responsible for it and for the security of your computer system. Valve is not responsible for the use of your password and Account or for all of the communication and activity on Steam that results from use of your login name and password. You may not sell or charge others for the right to use your Account, or otherwise transfer your Account, nor may you sell, charge others for the right to use, or transfer any Subscriptions other than if and as expressly permitted by this Agreement (including any Subscription Terms or Rules of Use) or as otherwise specifically permitted by Valve.