If you take a closer look at the Eula ( End User License Agreement) that you need to agree to in order to install and play HL2 you might put the CD/DVD back into it´s cover and return it to the store you bought it at. You got 2 pc´s and like to install the game on both pc´s ? - you better don´t . You played through the game and like to sell it now ? - you better don´t . Here are some snipets from the official Eula :

“If Valve gives you 30 days notice, they can make you pay more for HL2:
Valve reserves the right to change our fees or billing methods at any time and Valve will provide notice of any such change in at least thirty (30) days advance.”


“Valve can change the license agreement, and if you don't like the new agreement, you have to stop playing HL2 within 30 days”

“You can't sell a copy of HL2 second hand:
If you permit others to use your Account, you are responsible for their illegal or improper use by such persons. Your Account and Subscription(s) are subject to the Agreement. You may not sell or charge others for the right to use your Account, or otherwise share or transfer your Account.”