-Ask for part numbers for every single thing in that case and anything included.
-Find out when the seller bought that stuff, how it's been run (any overclocks), and what software is included if at all.
-smoking or non-smoking home; pets? (basically is it cleaned out often)
-invoices for the products if they're still warrantied
Basically find out everything in, about, and behind the PC. I highly doubt that PC has anything near worth what a MacBook Pro is worth. Also, if he gives you iffy, strange, or "political" answers, the seller is probably hiding something in the history of the computer. Most people of the respected hardware forums (Anandtech, HardForum) will tell you every small incident in the device's life; people on craigslist generally won't.
For a macbook you are going to want something with at least an i5 2500k or an I7. A core 2 quad is old technology so i'm guessing that most of the stuff in that rig is going to be old technology. I can tell you right now, w/o knowing what is inside that thing, that it probably won't be worth more than $500.
Enthusiasts definitely are no longer running core 2 quads. You would be better off finding a trade partner on OCN or hardforum.