CERF certifies that the eyes of the dog are clear. Von Willebrand's Disease Negative means that the dog has been screened for this disease. This disease is similar to hemophilia. Many temperament tests exist to determine the aptitude of puppies from obedience to hunting. If the breeder has conducted any temperament test, ask them to share the results with you.
Responsible breeders take every precaution to produce puppies that are free of genetic diseases.
If something unforeseen DOES surface, a responsible breeder will normally have a system in place to deal with your special circumstances. All of these details need to be discussed prior to buying the puppy. Responsible breeders accept a lifelong commitment to the dogs they breed. No breeder wants to see one of their puppies end up in a shelter. The breeder should commit in writing to take the puppy back if you are unable to care for it, no matter what age the dog is.