I would make sure he has the front fender and the left side mirror before I would consider it. You will want the fender, both to keep the rain or road moisture off you and it adds the the rigidity of the front fork.
Anyhow, the price seems fair, especially for the year.
Make sure it starts from cold and can hold an idle. Let it idle a little bit and check for leaks I'm assuming the 500's have carbs? Take it for a ride and see if it shifts cleanly through all gears and ride fairly slowly. It always seems that most issues show themselves under slow even acceleration. After you test rode it, check all around again for any leaks or damage. Check the chain, is it adjusted well, fairly clean? How are the tires?
My biggest suggestion, if anything seems off, either with the bike or the person selling it, walk away. There are plenty of used bikes out there.