We'll hope he wasn't loaded up with bute. If you buy him, make it conditional that you can return him within 3 days if he becomes unsound. If buted it will be worn off by then. Do ask the vet if the horse has been buted. This can account for a nice quiet horse. What I like to do is run my hands over the horse's back and when I get to the tail I lift it. Usually a horse will clamp it tight. A drugged horse will let you move it away from it's body.
He was not buted.