How to ensure an iPhone is OK to sell after fixing it?

Awhile back I bought a bundle of 6 broken/as-is iPhone 4/4s. I am just now getting around to repairing them.

After I make sure everything is fixed cosmetically (Screen, Backplate, midframe), and make sure all the buttons worked, the dock, headphone jack, speakers, earpiece, back and rear facing camera, vibrator, etc.. What else do I need to check to see if all the functions work correctly on the phone to avoid a unsatisfied customer.

I know check WiFi, Bluetooth, but what else?

Thanks!