I like to meet them in a dark alley around midnight with cash in my hand. I don't need to check the box, I trust them and it is pretty heavy, so I know there has to be a mbp in there 😀
Meet at a Starbucks or Bookstore or the mall. Make sure the original system restore disks are there, original charger, check for nicks and scratches(if you're an anal person), start it up, try several programs, bring along some dvd's and cd's, test playback of audio cd's, try to install a program from a dvd. Test out the internet access.
I.E. test it out, just like you would a used car. Bring a friend too, it helps to have a 2nd impartial opinion 😱