It always comes down to knowing your seller's reputation, you know, a horse dealer, or a small breeder that really cares and tells you anything and everything about the horse, good and bad.
What is important to me is that the horse is matched to the buyer, they know the horse's faults so that there is no surprises when they get him home. Every horse I have sold is a boomerang horse, if the buyer is not happy, bring him; back to me, I don't want him moving on somewhere else.
As for the dwarf issue, the possibility is there unfortunately waiting to happen in many lines, a toss of the dice when it will show up, sorry about your mare, wish you had known.