Look really closely for any hairline cracks. With a magnifying glass if you have one. I don't know which ribbon cable is the one for touch, but if you take out the motherboard it should be easy to trace the ribbon going from the touchscreen.
Does GameStop buy broken consoles? Even if they do, you would barely get anything for it.
If you mean getting it repaired under warranty, they will just send it to Nintendo's repair center who might be able to tell that it's been opened, and if they do, they will either refuse to repair it or charge a hefty fee. But I guess you have nothing to lose.