wyominggrandma is exactly right.. I've never sold a cow or horse without doing some thorough research on the people I'm considering selling the animal to-- whether its for $1,000 or $50,000. If the extra work isn't worth it to them, well, then they don't deserve to own my animal. Usually I'll call their vet, farrier (hoof trimmer for cattle), trainer (if they have one), and maybe I'll ask around about them at a show (if they show). You always hear the worst of the worst about people at shows so be prepared to take that into consideration.. Not everything will be true, but some of it might be. When possible, I also like to visit wherever the animal will be going to live..
$10,000 isn't small change to most people, so they should be prepared to show that they know what that horse is worth by working with you. But in the end, if it doesn't feel right, then it never is.