None of the other assets in the listing (screenshots, description, etc) are particularly revealing, although the box art and banner do look exceedingly legit. Obviously, none of this is official until Obsidian or THQ make some sort of announcement, which hasn't happened yet.
It might be possible that some rogue Microsoft employee, in a nefarious attempt to cause mayhem and wanton confusion, manufactured false artworks and an inaccurate Marketplace listing, but where's the motive? Was he spurned by a former lover? Did he do it for the money, for the fame? Nope, sorry, this screenplay just isn't coherent enough. Get out of my office.