If the camera comes with all the goodies, it is safer than if there is just the camera with maybe the charger (which could have been in a bag that was swiped). Receipt would be best. Just remember there are many craigslist scams going on right now, if seller states they are out of the country and payment can be arranged to be safe for you, run the other way.
I think I would require a copy of the receipt if I were going to buy something locally. At least get a feel for the person, like what photos have they taken and posted on the web, etc. If they have a 5D, then they most likely should have some internet references, you don't buy typically buy a 5D on a photographic self-discovery trek and then decide to sell it.