According to the Steam subscriber agreement, a person's account is subject to ban if they attempted to sell the account: "You are entitled to use the Software for your own use, but you are not entitled to: (i) sell, grant a security interest in or transfer reproductions of the Software to other parties in any way, nor to rent, lease or license the Software to others without the prior written consent of Valve."
When the user asked Steam support if he'd been permanently disabled, they replied coldly, "Correct, you lose access to your account permanently for violating our Subscriber Agreement." Thus, nearly $1800 worth of product was made inaccessible to him.
Bluesnews, the source of the story, attempted to contact Valve for comment, but failed. However, later in the day, a post on the Steam Users' Forums from Valve stated: "The article doesn't mention that the account has been re-enabled." No reason was given for the reinstatement, but Valve could conceivably have been trying to head off negative publicity.
Regardless, this raises a serious issue with services like Steam: you don't own the game. Rather, you're more or less renting it for an indeterminate time til Valve decides for whatever reason you don't own it anymore.
There used to be a time you could sell your boxed game to another person - box, manual, disc, and so on - and it was perfectly legal. Those days are over - at least until EULA's are outlawed.