At that age I would look at what kind of condition it was in. If properly maintained and low hours it would be worth the $500 bucks. If not maintained and beat up it is worth nothing. A quick visual check-starter rope,is it badly worn? The engine itself- oil coated and dirty or nice and clean? Is the throttle and tiller handle nice and smooth or loose as a goose? The shift lever move easily? ( sometimes you need to turn the prop by hand to align the gears) Ask the seller if you can take it to a reputable mechanic to give it a quick checkover. If he refuses -dont buy it. If he accepts, a few bucks checking it out may save you some money. Just a few things off the top of my head here but you should get the idea.