I work for Gamestop, and I can say that we honestly do not care if our used games have cases and manuals or not. Hell, sometimes we even throw them away. If they are in somewhat bad shape, we don't even bother. Also, if we have multiple cases for games we have one copy of... guess what happens to the other cases? The artwork and manuals are thrown out.
Besides, it's unethical to see what Gamestop does to customers.
For example, recently at my store, a customer had purchased a copy of The Bouncer for PS2. Thing is, we've had one last new copy for a couple of years now. Still retails for $39.99. Well, he wanted it new, instead of used for like $5. Point is, he waited about three weeks, and today he came up to me to return it. My manager was right there, so he immediately imposed the fact that we can only accept it as a trade-in. It trades in for $.75. The unethical thing is, technically, we can accept the return... but my manager did not want it in inventory again. Alas, we're not "supposed" to authorize the return, but we're allowed to.