The safest way is not to do it. The Humane Society recommends that people stop buying from Pet Stores and Web Sites, most Websites are fronts for puppy mills, and every time you buy a puppy you are condoning dogs to live in horrible conditions. Always meet your breeder in person, and see the conditions in which the puppies are parents are raised. If you ignore all my other advise, please be very wary of anyone who wants you to wire money this is ALWAYS a scam, and you will have no recourse. Have you thought of a rescue? The YTCA has breeder referrals and some of the breeders have rescues.
Yorkshire Terrier Club of America About The Club
Btw, how much is this dog, several hundred makes me suspicious it's a scam.