You shouldn't feel that you need to find every small thing that's wrong with it. Since it is a 13" MacBook Pro, it's still within the one-year limited warranty so you are covered. First thing I would check is to verify everything he's told you. For example, if the seller has a receipt, make sure that he does have it and it matches the computer he's selling you. Also check that the serial number on the box matches the serial number on the Mac. You might also want to check for any dents because that could result in your Mac being ineligible for service due to accidental damage. It also wouldn't hurt to have both you and the seller call into AppleCare and verify that everything is ok, at the same time he can transfer (re-register the computer) to you.