Check the actual serial number on the bottom of the laptop. The S/N shown in the software can be hacked. Also check for the specs - in about this mac - to ensure that all is correct. Do a verify disk in Disk utilities. Get the original disks that came with the laptop.
I never buy used computers, but if I did, this is what I would do.
1) Take it to an Apple store.
It doesn't matter if it has Applecare. The owner might have screwed around inside and voided the warranty. Only the Apple store people will be able to confirm that it still qualifies. Have the owner take it in (it's his computer) and ask them to confirm that the computer is still covered under warranty. If they won't (i think they will, but let's say they don't) he can make up a problem. Maybe he hears a strange noise from the drive or something.
2) Hook it up to the Internet
Make sure the wireless is fine, do some of the surfing you normally do, and see how it looks. You'll need these for step #3 too.
3) Run some apps
Put it through its paces. Start streaming a movie, turn on itunes and get something running, and maybe even run something like handbrake to really tax the processor. Take a look at iStat, see how things look, and listen to the computer. Any funny noises? Memory stats look kosher?
4) Sign a piece of paper (two copies--one for him and one for you)
This is basic stuff, but hardly anyone does it, even for major items. If he won't do it, then something is fishy.
http://www.documatica-forms.com/usa/computer-bill-of-sale/form.php?cset=yes
If something happens later, you'll have his address (get it off his drivers license), phone number, etc. And, you'll have written proof that will pretty much guarantee a good outcome if you have to go to small claims court. No paper = no help.
