I buy laptops off Craigslist all the time -- refurb them and list them on ebay /craigslist for slight profit.
Apple store will transfer ownership over to you with little hassle if you explain you just bought it off craigslist.
If I'm not sure about ownership, my test is whether the person knows the password for the computer. (Unless it's a clean install, and then I just want to take the recovery disks.)
Keep in mind that you can call AppleCare (1-800-APL-CARE) to check the serial number if you're in doubt about the ownership or whether the computer is still under warrantee.
Remember to get the remote, installation disks, passwords, power cord (with extension cord and duck), and case (if applicable). Original box and receipt are great too, but some people don't keep them and you shouldn't pass up a good deal b/c of it.
Also, even if it's still under warranty, AppleCare doesn't cover spill damage. So look around the keyboard / take out the battery and look around battery compartment for any spill damage.