If the used one has been taken care of, it should be a pretty good bike. Look for the obvious stuff like scratches where they should be. Signs of dents, the same. Check to see if the nuts and bolts have all survived (there could be some missing that he is not even aware of), and ask the owner if they have been loctited. Look at the battery and check the leads. See if he's activated the headlight switch. See if the spark plug has been replaced by the NGK. Check the chain closely. They have a habit of wearing out rather prematurely. Check the sprockets and see if they're cupping. See if it starts easily, as in quickly with the electric starter. After it's warm, try to kick start it. It should start in a kick are two. Don't use any throttle. Don't worry too much about that if it doesn't start right up if it started OK with the E/S. Ride it. It shouldn't vibrate very much. There will be some, but it shouldn't be uncomfortable. t
The peak of the vibes should come about 62-65 mph (100 kph). the suspension might feel a bit harsh, but that's the stock animal. There are ways to take care of that as with Hagon shocks and fresh front fork oil. The bike should run quite smoothly throttle wise, and the front brake should be quite adequate. The rear one can be, but there are a lot of them out there that aren't properly setup. You can take care of that. Even the new ones can have that problem.
As a C5, check it in the 60-70 mph range for front end stability. Some of them are noted for getting a bit of head shake going. Look at the forks. If it has the leading axle, they are more prone to shake. It it has the straight sliders with the axle inline with the sliders, it should be OK. They changed the axle location in the late 2011 production run. Except for starting, any heavy clanking should be a giveaway of something amiss. At starting, though, particularly when the engine is cold, the decompressor can clatter for awhile as the engine warms up. That one is OK.
I'm sure more will chime in with other stuff for so I'll let them have the floor!
Bare