For almost all LCD screens, 3 or fewer dead/stuck pixels are considered an acceptable manufacturing tolerance and are as such not covered under warranty.
Problem with buying at a game store is there's a higher chance that the employees would know that than if you bought it at a big box retail store. At those places, all you need to do is walk in and say "Yeah, the screen on the gamepad is defective" and if they press further, "a small portion of it doesn't work". What are they gonna do, plug it in there and try it?
Of course, you could also definitely go and try it at gamestop too, but the problem right now is that nobody has the WiiU anymore. They're all sold out.