That's what I thought at first... and then I watched some MBP un-boxing on youtube. They are wrapped in plastic... units sold in 2012 have a white sticker seal on the back.
Gotcha. The problem is the risk of bad outcome is significant, and your entire investment could be wiped out.
If the seller was legit, there's no reason why he would refuse to let you open and inspect, if you said you would meet him in a public place like Starbucks, show him that you have the cash on hand, and assure him you want to buy but just need to make sure it is working and as described.
If they refuse that, then they are either a scammer or just seems fishy in some way.