I agree with the others as to his hind quarters and his coupling. Because the horse was out at pasture and this is just one photo I will not say his camped under hind stance is due to any problem... it may just be what the photo captured.. as he was moving around.
That said, his front let is over straight and his front feet are low heeled and long toed which is not helping is overly flat pastern.
This is a 4 year old QH and that is quite a project to take on. I would move on and find a better horse which should be very doable in this buyer's market.
A good horse takes as much effort to train as a not so good one.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer