The best way to sell a purebred dog is to an official breeder of that specific kind of dog, as selling the dog through other means is considered shady and even illegal. Dog breeders have licenses and training and tend to screen their customers thoroughly first, so they know best.
Selling a purebred dog is a practice that requires a great deal of research and thought. Some states have strict laws about the sale of animals, particularly dogs and cats, and tend to frown on illegitimate establishments or attempts to sell to other states. The American Kennel Club has a long list of guidelines, as official certifications that can be earned, that help someone qualify for selling purebred dogs. However, a breeder is someone who tends to focus on one or two breeds of dog only, as the process is very intensive.
Officially certified breeders are the best people to sell a purebred dog through because of years of experience and study in that breed. They know how to identify a purebred of their specific breed of dog with ease due to years of raising them and they are familiar with the laws and regulations relating to the sale of purebred dogs.