Welcome to the Forum. Hope you become permanent.
$500 sounds like a very good price if it's running well.
In our market, a really good runner, which can pass the mechanical inspection and which doesn't need major parts like mufflers, tires, sprockets, chain, brakes, cables ... can command $1000 +/-. It also depends on the mileage - how many on this one?
If this bike doesn't need any or much of that right away, then it is a good deal. If you are not sure, search a couple of parts places for prices, add what you think it will need to the $500. If you get north of $800 and are still counting, take a deep breath and sleep on it.
Oh, don't forget to count the cost of safe handlebars and repainting the tank