Get as many photos as you can of the box to determine whether it's an official Wizards plastic seal, and not significantly torn. I personally haven't seen any counterfeit Wizards wrapping, so that's usually a good sign that it's legit and not full of weighed packs. If it lacks the Wizards logo plastered all over it, it's resealed and/or fake.
$600 (I assume USD) is a decent price for a First Edition box (another thing you need to check). I personally would try to talk him down a bit if he's willing, otherwise I wouldn't feel too ripped off at that pricepoint as it seems to be the typical going rate for a 1st Ed. Jungle. If it's Unlimited, I would personally spend no more than $500.
Also note that with these old boxes, if you intend on getting the cards graded for resale, you'll most likely get back at least the amount you spent if they are indeed in mint condition. But if they have significant wear or silvering, you'll fall short in the end. It's sort of a crap shoot with grading the classics.