What to look out for when buying iPhone 4?

[JasonChiang]

What should I look out for when I buy an iPhone 4 off craigslist?
It is just the phone itself, with no other accessories and the guy claims the phone to be mint.

Make me a checklist please. So far I got:

water damage
scratches

[805 93003]

Majority of iPhone 4 problems can be determined with just making a call. Will check SIM reader, antenna, earspeaker, microphone, proximity sensor, screen function (check all the numbers), external speaker etc. Also check Home Button, Power Button, Vibrate Switch and motor function. Check WiFi if you can.