I want to give them the benefit of the doubt and say maybe it was a matter of miscommunication because the correspondence was over email. However, when I asked them what the process was, they did not mention meeting parents or any sort of contract. They just mentioned putting a deposit down and when I eagerly said I can come as soon as possible to put a deposit down in person, I was told this wasn't possible because it would stress out a momma dog they had in their house.Well I can see not allowing people to meet a litter that is less than 4 weeks old. Momma dogs do get protective of their babies, and introducing strangers to the momma + young babies would not be a good idea. But what do you mean by "momma dog with a previous litter"? If it is a previous litter (not the litter you would be getting a puppy from), you do not need to meet that momma dog. But you do want to meet the momma of the litter you are interested in, either before the pups are born, or after they are about 4 weeks old.
Getting a puppy from a momma who has a wonderful stable temperament -- one who is not easily stressed out -- is extremely important. Mommas pass their temperaments along to the puppies in two ways: (1) genetics and (2) by taking good care of them (the breeder and the momma dog together provide the puppies with a safe, secure and happy environment). I think you are wise to be cautious.
I can respect their reasons 100% but I did not feel comfortable sending a deposit to someone I don't know without too much information.
Hopefully, other people have better luck with them.