Most software companies have EULAs that declare that digital purchases are licenses rather than products, barring users from being able be resell digital copies of software.
Recently, the European Court of Justice showed that it disagreed, ruling in a landmark case between Oracle and UsedSoft that "[a]n author of software cannot oppose the resale of his 'used' licences allowing the use of his programs downloaded from the internet."
Such a ruling applies not only to Oracle and UsedSoft, but to all software companies. Unfortunately, those hoping to resell their purchased Steam games, because of the ruling, will be sorely disappointed.
"We don't have any plans to change," was the curt response that Valve business development director Jason Holtman gave after being questioned on the issue.
The ruling may allow users to sell used licenses for software, but that doesn't necessarily mean that Valve has to make it easy or possible for you to do so. I'm assuming there'd have to be another ruling of some sort to make that, if it ever will, happen.