Can't really tell in the pic. Is the front sight the tiny little "bump" style, or is it more raised with a dot? The rear sight looks like it belongs with the taller dot sights....
If it has the taller dot sights, it's the milspec, and worth between 400 and 550ish, approximately, depending on condition.
If it has the teeny tiny hump and bump sights with no dots, then that one would be the GI model, and worth slightly less.
Either way, if this is a potential purchase, try not to pay more than 500. If he wants more than 550 or so, just save a little more and go buy a new one at around 650.