I have heard many stories of people being ripped off via Craigslist.
Use common sense. Check to make sure it works properly. Test speakers, test the switches (home button, volume up/down and rocker switch) Take a pair of ear buds to check the head phone jack. Be suspicious if the battery is dead so you cant test it. If the box is sealed and, do not give him any cash until you open it to verify there is an iPad inside and not just a box with a brick! If it is in some kind of case, remove it from the case and visually inspect the entire iPad for damage, dings, scratches, etc. Plug in the charger to see if the unit shows that it does charge. (this tests both the charger and the charging circuit inside of the iPad)
And be wary of someone looking to rob you. Don't meet in a secluded area. Take someone with you, preferably someone large and mean looking. Hide the cash in your car until you come to an agreement, then retrieve the money from the hiding spot.
Good Luck