I sold a brand new unlocked iphone on ebay. Before I mailed it out. I tested it. It worked perfectly with my T mobile sim. Then the buyer told me it did not work for his at&t net work. He said he went to the apple stores to fix the phone and failed. He asked for refund. I told him to mail the phone back and if the phone is still in the same working condition. I'll give him the refund. Today I got the phone. I worked with the phone for several hours and couldn't start it. I'm afraid the buyer may did something wrong on the phone. What should I do now? Any suggestions? Thank you!