Stolen Mac? Stolen hardware Registration site
Help to prevent the trade of stolen Macs: You can register your hardware as stolen in Second Hand Mac ‘Stolen hardware Registration’.
Check the serial number of the hardware before you buy. You should find it on the label, often under the computer or in the battery bay of laptops. You can get that number from System profiler reports( Applications->Utilities) on some models as well.
Contact the owner and the police if the hardware is reported as stolen.