You will register the game to a Steam Account. You could sell the game by selling your steam account, but the game will be tied to the steam account you register it to, so you would be unable to re-sell it individually (unless you made a steam account only for this game).
If you buy Fallout: New Vegas retail, you will get the CD-KEY. You will open steam and will go to Games > Activate A Product On Steam and enter in your CD-KEY. This will bind your copy of Fallout: New Vegas to the Steam Account you are logged into. It is permanently connected to said Steam Account.
I'm mostly referring to console games, where a growing trend is the requirment of a one-time access code to play certain games. Right now, it's to access mostly online features.
Also, I am aware EA doesn't have anything to do with New Vegas; I can see how that was missconstrued in my comment, though. I was merely using them as an example as they are the biggest example of this sort of thing.