When you go to make the exchange, meet in public like at a Starbucks. Even if is sealed box, make sure you open it, turn it on and view "About this Mac" to ensure is all he says is.
I know a guy who met someone who sold him a "sealed box" MacBook Pro. Paid $1550 cash. He took it home and opened it and was no MacBook Pro in the box but rather a rock. Said it was perfectly sealed so he assumed was legitimate.
There are a lot of scammers out there. But there are also a lot of good deals from honest people who have just changed their minds for whatever reason and don't want and just want cash to buy something else. The whole thing is be very careful. See it, keep it in your hands, then pay cash for a safe transaction. An honest seller will have no problem letting you test it fully.
Good luck.