Definitely ride it and see if you can't get a mechanic to look at it.
You said the guy you're buying it from is the 2nd owner ... you have no idea how the original owner rode the thing. Having the guy say that he wants to be able to buy it back sets off a BS alert in my mind.
You probably won't be able to drop the motor and look at the internals, but just check basic maintenance things like brake fluid (color and levels), cleanliness under the fairings (no spilt oil from the filter during the last change), brake pad where, hose/cable condition, etc. There are parts that need to be replaced every 2 or 4 years instead of based on milage(fuel line, brake lines, clutch/throttle cables ... things that just get old and rot), so check those to see if the current owner has been keeping up with that.
Just my $0.02 worth.