I really tried to just look at the horse and not the rider... But it was a little hard haha.
I'm not one to ask for confo. critique, but I do know a thing or two on the riding aspect :)
Personally, you're getting ripped off for $12,000. That rider must be SERIOUSLY tall if that horse is really 17.1 or 17.2. (Mines 17.3, I'm 5'8 and I don't make him look short by any means). He looks unhappy to me, and strung out. Training could fix it, but who knows. IMO, he wouldn't be my type and I would keep looking. Rider error could have made a "deeper mark" on him which could be a hard fix. Never know until you try him out though :)
If I was you, I would go check him out and ride him for yourself. Ask why they're selling and see if you like the feel of him. What you can tell just by this video is that he is obviously a very forgiving horse (considering his rider) and looks like he is trying to do what they ask of him.
Good luck, I hope everything works out great for you :)