I would tell the seller at my trainer's visit, after she has indicated her good opinion of him, that I'd schedule a PPE as soon as I could and I'd let her know the minute I had an appointment. If your trainer indicates that he's not for you, I'd also let the seller know that ASAP too, it's only fair not to tie the horse up any longer than you have to. Have you put a deposit down on the horse? I ask this only because when I am selling, even with an accepted offer, if you have not put down a deposit I will not hold the horse. If you have put down a deposit, if he fails the trainer's ride and/or the PPE, then I refund the deposit right then. If you just back out, then the deposit stays with the seller. Otherwise, she/he is within their right to sell to someone with cash in hand.