Scams are everywhere. Be careful ask for photos with a paper with your name on it to be sure the pup is real. Call the person!! ask knowledgeable questions and know a bit about the breed so they don't just make stuff up.
Not all breeders will allow you into their home. After all you are a complete stranger and dog theft does happen. People do come to check a place before coming back to rob it. So I understand the no visitors. But If they cant send you pictures of the pups playing in the home, in the yard, or the pups are always in a kennel with mud or dirty that is a red flag.