You should emaileBay and ask them about it. They have a department dedicated to scams and spoof emails.If this is a newish thing going on, they may be able to tell you that it's a new scam, or they may direct you on what to do. I would consult them and the police.
Also how does the recipt this guy has, have the serial number? I've never heard of that.
If it IS his ,aptop he should have the ORIGINAL factory documentation. I would DEMAND to see that, not some reciept that could have been drafted up in Word in 2 minutes.
