Meet in a well lit, public place with wi-fi. Tell him you'd like the serial number in advance so you can check it on a website that lists stolen macs. Ensure he's listed as the owner of the machine. (Open up terminal and see if the owner is listed...ie, Seller First Name-Seller Last Name - type of machine-)
Ask him WHY he's selling it and see if it passes the sniff test. I bought a steal of a deal on a 17" late 2011 macbook pro last July on CL. Guy had all the box.,etc, gave me the serial number to check, and informed me his aunt bought it for him when he entered community college, but he dropped out and needed it to fix his car.
Got a base model 11" 2013 Air for $800 (less than two months old), but had to drive 30 miles on a CL ad. Had all the boxes, etc., and it was provided to him for a computer science course. He used it for the time he was required to, and knew he wasn't going to work on Macs for his job.