I've never run into this problem with GBA games since I bought all of mine new and sealed, but I remember people whinning about getting fake games before on the gamefaq GBA boards.
I wouldn't have a problem with it if they weren't ganking the GBA slot, but they are, and then trying to re-sell the games. It's like Nintendo saw how lucrative the GBA market still is, and now they want to screw us and charge us for the games again. I know I had zero interest in GBA games until I got my DS Lite. With the access to the better DS screen, I was able to play the GBA games that I wanted to try, and I'm sure this happened to many other people as well.