Let me ask you a question. Was this a regular retail purchase, or was this an open box purchase? If it was an open box, it is possible the device was activated prior to your purchase and was not wiped correctly, so it still had the original purchaser's Apple ID on it. It this was a regular purchase, then I'm not sure how it could have been hacked to put someone else's Apple ID on it for Activation Lock. Since Best Buy is an Authorized Reseller, if they are not providing a receipt that displays the information that Apple requires, then you can take that up with Apple so they ensure their authorized resellers meet their requirements. As far as discussing the point here, we are users like you, and Apple does not participate here. Nothing anyone here could do to change your predicament. If Apple states they will not do anything without a serial number receipt and Best Buy refuses, or cannot provide such, it is what it is. The device is Activation Locked, and you cannot get in. Sorry for your loss.