Feedback for those who get a similar proposal in the future:
Some people perform insurance claim scams where they report their MacBook Pro or other laptop stolen. Their home-owners insurance demands proof of purchase and some times a receipt isn't enough. After all, it would be very difficult to go through the proper channels to get the proof of purchase.
This is especially true if you bought the laptop second hand or from a seller that isn't easy to get ahold of to prove purchase. Apple has a help line, but what about Wal-Mart or Target?
If one were to report their MacBook Pro stolen, having a receipt and the box would help with the argument that it was snatched out of your car or book bag.
When a MacBook is reported stolen, a reimbursement check is cut for the full retail+tax value of the MacBook and in the process, the MacBook's serial number is reported to the police department.
Where does this put you?
The Apple store will not have any knowledge of it. BUT, if you turn your laptop in for repair, the name on the MacBook will match the name registered to the laptop. There's a chance you will not be given the laptop back when repairs are done because you aren't the person who bought/registered it.
However, let's say you get pulled over by the police for a speeding violation. The officer sees your MacBook in the back and asks if you'll let him take a look inside. If he has any reason to suspect the laptop is stolen he can run the serial number and it will be flagged as a stolen laptop. You will get arrested on the spot and charged with possession of stolen goods over $1000 in value. This is a chargeable offense and you WILL have to be bailed out or serve time until trial. "I didn't know it was stolen" is the worst argument they receive and will do worse to get you released. Even IF you promise being able to furnish proof that you bought it off someone, you would have to provide that proof to the detective who arrested you who will send someone to ask questions. If he can't find that person, you're SOL.
Taking possession of a laptop when you're not sure whether or not they'll report it stolen to perform such a scam is a risk. Is it as big of a risk as driving a stolen car? No, not really because the proper methods in place to minimize those scams are very minimal, at best.
How do I know this stuff? My friend's MacBook was actually stolen and in the process of trying to get it back, we went through all channels. Police department is the ONLY place that actually records the serial number and keeps it in a database. Apple bluntly said they don't flag or keep a database of stolen MacBook Pro serial numbers.
This would explain why this seller was attempting to keep the box from you and he doesn't seem to really care if you know who he is, etc.