If you are buying in a cold climate, check for rust! The inside of the swing arms can fill with water and rust, you can remove one of the chain tensioners and take a look inside. The outside of the swinger arms can rust. Wheel bearings wear out after 10,000+ miles. The exhaust catalyser (box in middle of the exhaust assembly) rusts in colder climates, this is really expensive to replace upwards of 600 pounds. The engine bolts and fairing screw can also rust and need replacing.
The bolts that attach the exhaust pipes to the motor (four) can rust and be very difficult to extract.
Advise, treat the bike with an anti-rust preparation and then regularly every winter.