Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148 Safari/6533.18.5)
It sounds like the problem that your buyer is describing is a dead short creating a boot loop. If he is scamming you it is possible that he has swapped a shorted component(such as an LCD or top ribbon) or worse the main board. If it's the main board it will be easily detected as the serial number will be different under Settings>General>About. Other components of the phone are not numbered which would make it impossible to detect other than the "do not remove" sticker being broke. Anyone with this expertise would probably understand how inexpensive it would be to just repair an individual part. The value of an iPhone is the main board. So if the serial number matches when you get it back(in the about menu, not on the case) then your phone has probably developed a short somewhere, possibly aggravated in transit, and these are usually easily diagnosed and repaired by any reputable shop.