Since the majority of posts in this thread are entirely unhelpful: no, it isn't.
Firstly, you have the right to sell any physical copy of something you have bought. This is true of objects far more expensive to manufacture than a single copy of a video game: electronics, cars, private jets, cruise ships, and so on. People don't complain about the App Store because:
a) You have bought a digital product, not a physical one.
b) It cost almost no money to purchase.
Those two things are true, to a lesser extent, for Steam.
Secondly, someone on a tight budget is not going to be willing to spend as much on games if they don't think they will ever get any money out of it afterwards. Consider that you buy a game for $60, thinking that you can resell it for $20, and you will have paid only $40 for the game. At that price point, you will be more willing to buy any given game, and may buy more games in general. Microsoft could, of course, cut out the middleman and just cut the MSRP to $40, but that would require the agreement of retailers and publishers and so on, so it's a more difficult solution. Plus, this way anyone who wants to "spend that extra $20" as such and keep the game gives the full $60 to Microsoft/the devs/the retailer/etc.
Thirdly, you've seen the backlash whenever anyone tries a policy of this sort, and that kind of publicity will not help sales.
(Former) Lead Moderator and (Eternal) VGC Detective 
