As a trainer myself, I would much rather deal with a horse that bucks, takes off, bites, has horrible ground manners, than one that rears. IMO its much easier to "cure" a horse from bucking, and much less dangerous than it is to "cure" a horse from rearing. Once or twice because of pain, or misunderstanding because I'm not asking something right is okay, I'll check it out, but if the horse has flipped over, even if he only reared once and flipped, definitely not, and if it rears every time someone rides it, then I would most definitely not.
I'm currently working a 6 year old warmblood mare, that I believe was pushed to far too fast by the owners daughter (daughter has passed away, so I get bits and pieces of the story of what really went on), and she started by just stomping her back leg, twisting her head around, gnashing her teeth, and not going forward, then once she started going forward, and bucking/kicking out wasn't working to get out of work, she resorted to going up and almost over twice, and I SERIOUSLY got after her for that, and if she does it one more time I am going to have to politely tell the owner that I can not train her horse because of it. I will not put myself in that sort of position, and I am a little shocked that the owner is trying to sell the horse as ridable even with her disclosing that he is a rearer. I would get the trainer involved, let him know whats going on, and have him talk to the parents himself, but outside of that, I think that you have done everything you can to warn them off the horse, and tell them how dangerous it is. Its sad that they aren't listening to you, but unfortunately not to be harsh, they may need for the daughter to get injured before they will see reason, though even then they may just blame it on the horse. Even my parents who aren't particularly horsey people wouldn't have EVER allowed me to get a horse like that, and even now that I'm 26 and have my own training business, they would highly discourage my getting a horse or training a horse like that.