Is there even a key or something unique that users could exchange for most games on steam that would make a resale possible?
At the moment I don't think there is a way to do that.
But it's probably in Steam's interests to make it happen, from their own platform. Otherwise I could write a tool to do that, start my own website "occasions-vapeur.fr" and charge 1 EUR for each sale and it would presumably not be illegal to operate such a brokerage website in France (keeping in mind that reverse engineering for such purposes is explicitly legal in Europe).