I have been looking on C/L for an iMac and found several. However, some say they don't have a receipt. When I plug in the S/N on Apple Support, is says "unregistered, please submit proof of purchase". I'm not about to support the local theives, however, is this unusual, or are these hot off of someones car seat? If it is legit, and the seller just never registered and doesn't have a receipt, is there any way to register it?