that they bought new, but opened up?

I recently bought a supposedly new copy from an Amazon seller recently and opened it up today and notice that there is a glossy "discoloration" (It looks like it could be a stain) next to the gba game holder inside the game case; I couldn't take a picture of it, but I edited a ds case picture I found online to show where the discoloration (See below). The area I am talking about it outlined in yellow.

My DS is being repaired so I can't check if the game cartridge has any old files on it until I get my DS back.

But, I've had problem with marketplace sellers passing off used games as new before. And since I overpaid for this game, I want to make sure I didn't get ripped off (I know there is probably no 100% surefire way to do this, but I can try).

Could anyone with a copy of this game take a look at there's just in case this is just intrinsic to the case itself? Thanks!