lucast73 has a point, those non-disclosure agreements only work if both parties can afford to enforce them. A bad hoof is usually pretty obvious to a vet or even a sharp buyer once the horse moves around a bit. Covering it up means the sale price is based on a lie, and that deal structure cameronwest mentioned falls apart when the truth comes out.
Depends how you structure the deal and the paperwork. Plenty of clean sales happen with non-competes that hold up just fine. People only hear about the messy ones that go to court.