I’m actually not sure about that; I think @Fraggles once explained that we do actually buy the games, but it is on the games’ publishers to enable us to play them, and as long as Steam works, they are honoring that obligation, but if Steam were to disappear one day, then it would still be on the publishers to enable us (direct download or a free key for another launcher or whatever) to use our product.
So IF i understand that correctly, Steam offers this service, and we agree to it; however, we do own the game, but the honoring of that sales contract falls on the publisher, not on Steam, and now (1 judge in) France (let’s not forget it’s merely a ruling by 1 judge atm) ruled that that is not how this works and that Valve have the obligation to allow us to sell our games (given that the ability to transfer ownership is a major condition for ownership itself).
If someone is willing to buy them, you can actually sell them, but they won’t be able to use them as they are a single-use item; this is not a valid analogy 