The only thing I would worry about is the fact that he's jumping already, at three years old. He shouldn't be jumping until he's at least five. Plus, the seller says he's raced before, most likely at two. He's been predisposed to unsoundness and he might not be ride-able five years down the road.
He is cute, though, and seems very sane and calm with a nice even trot and canter. He doesn't seem flighty or headstrong in the least, which is could be expected from an OTTB. I think he'd be even better with a decent rider. The one in the video was way ahead of the vertical, even at the trot and just took away from the horse. He's a really decent price, but again, you have the prospective unsoundness to consider.
I'd go see him. I agree about him being jumped too early but he seems like a really nice horse and as long as you vet check before any kind of purchase it should be fine. And yes, with OTTB's there's always a concern for unsoundness in the future, but I must also say that some of the best horses I've worked with have been retired racers. I at least think he's worth trying out though, seems like such a lovely guy.
I noticed the price has gone from $2000 to $1500 to $800. Bet you could walk away with him for $500 easy. He had to be registerd to race. He is very cute. I love his attitude. He is nice and steady but not exactly a flashy mover, seems like he will be a decent jumper. I would check him out if you are looking for an affordable young horse to bring along, he appears very versatile and calm. Def get a vet check of course.
Good luck and let us know how it goes.