..but supply and demand does play a large part of why Gamestop can set such a high price-point and most likely get away with it. Yes, it's true that Gamestop was the exclusive retailer carrying it besides Nintendo, but that doesn't change the fact that there was limited quantities available at time of purchase, there was demand for the product spurred by the good reviews/press it got, and then lead to $400 used copies floating around on the internet.
Assumedly, Gamestop saw an opportunity, either "found" extra copies or had additional pressed, and marked them as used because they can. They can fix prices any way they'd like on used copies, and once Gamestop purchases copies from the manufacturer, I imagine they're well within their rights to sell them as either "new' or "used", whichever nets them the most benefit.
In turn, as much as this is ruffling everyone's feathers, more supply will reduce the resale value of these games, so they are inadvertently helping those who might consider trying to find the game down the line.
At the end of the day, the anger should be directed towards Nintendo, not Gamestop, for encouraging this type of behavior. Gamestop's behavior is a symptom of Nintendo not producing enough of the product to meet demand, and then themselves not jumping on the opportunity to release pent-up demand.