I know you want to keep the two you have but on the other hand I don't know that I would want my horse to go to someone who would want to get rid of it after one buck. Even though you and the former owner never saw the horse buck it is 10 and you are only accounting for 2 years of its life. I agree they probably rushed the horse along instead of letting it settle in. So what kind of horse people does this make them?
Very possible someone hopped on the horse gave him a swift kick and hauled on his mouth. (who knows) Bottom line is they don't want your horse or don't want to give it a chance so I would say safest thing maybe to swap back. Otherwise it could get really ugly and I hate to think what might happen to your 10 yr old QH.
Just my humble opinion.