I've never seen ATV tires as bald as those before, whoever is selling this clearly rode it hard and never did anything in terms of maintenance, or even basic care.
He says he doesn't know what's wrong, blew the engine and he thinks it's the bottom end, if he's right then both bottom and top end are going to need overhauled, which is not as easy or as cheap on that motor as it is on a Blaster. Also it is highly likely that there is damage to the case internally.
Given the careless condition of the motor and the tires, it is likely that the brakes, bearings and bushings have as much life in them as the tires do (none). Also, judging by the way parts are just laying around he's probably already lost a few things.
If you're somebody who's very comfortable with doing a complete teardown and rebuild of a 4 stroke at home on your own, then this might be a decent project, but if you're planning on paying someone to work on it, there's no way it'll be worth it by the time you get it into riding condition. It looked like a steal at first glance, but really it's just a deep deep money pit. That's just my 2 cents on it. Good luck with whatever you decide.