I buy/sell macs on craigslist all the time. I make sure I meet where there is an internet connection so I can look up the serial number for its warranty status. As long as it is in warranty, I check for physical damage and make sure everything works: screen, wifi, bluetooth, usb ports, trackpad, keyboard. I also use System Information to make sure I am buying what I think I am buying.
If everything checks out, I buy it. My experience has been that as long as it is under warranty and not physically damaged, Apple will fix things that go wrong. I do not know of any way to check if a machine is stolen. So far, I have not hand any problems in that regard.
Of course, I ask if it comes with the box, receipt, both plugs for the power cord, but I may still buy it if the answer is no to any of those.