Just how much value does trading used games at Gamestop have over other avenues? Gamestop's entire business model relies on the trade of used games at rock-bottom prices so that they can then sell them at about a 500% (or higher) profit. Of course they sit on old stock for a while, but even at their fire-sale pricing for really ancient stuff they'd typically be breaking even.
The biggest problem is not that Gamestop offers the ability to trade used games for (credit towards) new games; The problem is that Gamestop offers incentives to trade new releases back as soon as possible, and then - crucially - turns around and immediately offers them for sale while undercutting the new releases, which does indeed hurt developers. While Halo 3 did phenomenally well when it launched, I personally saw many used copies available at a local EB Games on the shelf right next to the new copies on launch day. This is like making a home release of a movie available while it's still in theatrical release - It undercuts profits at a critical time.
I've said it before, but what needs to happen is to regulate when used copies are available on store shelves. Used games in themselves aren't evil, nor is the ability to trade back to the store early on. However, the way Gamestop and similar companies operate by making used copies available for sale immediately and advertising them alongside new (especially considering that a used game is, in theory, no different than a new game at this early stage) is the major driving force behind anti-used game tactics that publishers and developers are beginning to make use of.
If Gamestop wants its business model to continue without alienating developers like it has been, and without having to fight anti-used tactics like have been deployed recently, they need to step up to the plate and offer some kind of compromise on street dates where the "premiere" of a game is off-limits for used sale. Otherwise, the push for single-use digital distribution and locked-down hard-copies will only continue at an ever-increasing pace.