That doesn’t sound like a great experience for you or the potential buyer. If I was a buyer and got that kind of report, I’d actually be mad about it if the vet can’t provide any reason other than lack of muscle for the conclusions. Unless they think the horse has some kind of disease causing muscle wasting or something like that. But at the end of the day, it’s up to the buyer to determine if they want to take the vet’s advice or not or pay for a second opinion or not, and sometimes badly performed PPEs do kill a sale, but it may or may not be anything actionable against that vet.
I had one friend selling a horse, and she complained that the PPE vet spent way too much effort on front fetlock flexions, and ultimately the horse flexed a bit off and it killed the sale. I’ve had vets tell me personally you can make any horse lame just about if you do flexions badly, with ankle flexions being a common example given for overdoing it. But if the buyer isn’t experienced enough to see that was maybe one flexion too many or who would move onto xrays, then that’s just one of those things that happens in horse sales.
This is why it’s called “practicing”. Another friend of mine was selling a high end horse, competing at a top level. Never had any “maintenance” or soundness problems. One vet found something and said the horse wouldn’t possibly hold up. Killed that sale. Different buyer used another practice. Both well respected practices. Second vet said it was not an unexpected finding for horse’s age and mileage and that it might require some maintenance but would be an easy thing to manage and they’d be happy to help with it if it ever became symptomatic, which it wasn’t at the time of either exam. Same issue and two totally different opinions from two sport horse vets.
I would just move on and find another buyer.