Some things to look for....
Look at the milage in contrast to the bikes condition... was it rode hard and put away wet (scratches, condition of tires, condition/cleanliness of oil, condition/cleanliness of coolant, does the seller have a record of maintenance that shows oil changes, coolant services, brake fluid/pad changes, clutch in good contition? does it slip? reciepts for any work done or parts replaced? DOES THE BIKE HAVE A CLEAN AND CLEAR TITLE?? and yes i agree bring yourself a friend that knows what a bike is all about and is licensed, so he can take it for a test ride if possible.
big question WHY IS THE PERSON SELLING IT?
much the same as buying a used car...buyer beware!