Even though Apple makes some great quality devices they are not always perfect.
I finally found an iPad mini for store pickup tomorrow and I will make sure to open it before I walk out of the store. I suggest you do the same when you find one. A good checklist is:
1. Make sure everything that should be in the box is in the box
2. Make sure the condition of the charging wire and the actual iPad mini are flawless (no dings on the iPad mini, no kinks in the wire).
3. Turn on the mini. Look at the screen and check the volume. (if you are worried about getting a bad screen, you can compare your mini side by side with one of the Apple store's display units.
4. Ask an one of the Apple store representatives if they have an available outlet. Use this outlet to make sure the iPad mini can receive a charge and the lightning cable charges.
5. Last but not least, use the iPad mini to log onto a local wifi network in the area. Then use safari to navigate to a deap pixel checking site. Save the images and view each dead pixel checking image in full screen and at least half brightness. (this step will check for dead pixels as well as make sure the wifi works).
After all this, you can leave the store knowing you have a good working iPad mini.
-edit: here is a dead pixel check site:
http://www.theipadguide.com/content/ipad-dead-pixel-test-how/7171269