I'm in the US and will soon be selling my factory unlocked iPhone on CraigsList. Does anyone have any tips that I can do to prove it's factory unlocked? I'm on AT&T and don't have another SIM to try out - where is a good, cheap place to get one? I haven't had a chance to try out the unlock myself as it's a fairly new phone. I'm not worried about the phone being actually unlocked - I did purchase it from the Apple store myself - but I just want to double check things before selling just to make sure. Any tips would be great!