Asking for a phone number won't necessarily work since they could have you calling a friend that's involved in the scam. If you do get a phone number also ask for the name of the vet and do a cross check using
Yellow Pages Online Phone Book Local Business Listings Directory | DexKnows.com ™ to get proof that you are actually calling a vet. Also, if the owners are out of the country and are selling the Yorkie because of the weather being bad for dogs then it's a scam.......
Do not under any circumstances give personal info..... name, bank accounts, credit card numbers...... don't mail them a check since that's all they need to drain your checking account.
Good luck.