Taken from the official Steam license.
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).
A. License Terms.
Steam and your Subscription(s) require the automatic download and installation of Software onto your computer. Valve hereby grants, and you accept, a limited, terminable, non-exclusive license and right to use the Software for your personal use in accordance with this Agreement, including the Subscription Terms. The Software is licensed, not sold. Your license confers no title or ownership in the Software. To make use of the Software, you must have a Steam Account and you may be required to be running the Steam client and maintaining a connection to the Internet.