"When you complete Steam's registration process, you create a Steam account ("Account"). Your Account may also include billing information you provide to us for the purchase of Subscriptions. You are solely responsible for all activity on your Account and for the security of your computer system. You may not reveal, share or otherwise allow others to use your password or Account. You agree that you are personally responsible for the use of your password and Account and 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."
Personally I think that it's not fair at all, and you have the right to sell it. However it is against steam's rules. I tried selling mine on eBay once, not realizing it was illegal, and then a few days later eBay removed the listing.