HOW DARE YOU TRY TO SELL SOMETHING WHICH YOU BOUGHT AND OWN! Don't you realize that poor valve is struggling to survive due to such massive competition from the second hand market?
::sarcasm mode off::
Seriously, a license agreement is yet another way that greedy companies try to exploit the public. Shame on you, Valve.
And to the people who parrot the "Vote with your dollars": Yes, go ahead and try to vote with your dollars. Take a look at almost every software EULA and you'll see that the publisher/developer says the buyer isn't allowed to resell. So forget about playing any games. You can also forget about using almost any other software for that matter.
And what if you don't like the fact that companies are outsourcing all their production to China? Do we simply keep quiet and vote with our dollars in regards to that as well? If so, then we would be clothless; shoeless; void of any electronics of any kind including computers, tvs, stereos, phones, cameras; no more appliances such as stoves, refrigerators, microwaves, etc; we wouldn't own almost anything made out of plastic; no more cookware and silverware thus forcing us to eat with our hands; no cars (many parts to "American" cars are made overseas).