
It is possible that this person might be a puppy farmer. Most breeders these days tend to have one or two bitches and only breed to continue a certain line, which they have spent years developing. I do know of some breeders who by circumstance rather than design have two bitches they want to use in a breeding programme being mated at the same time more or less.
If many local people are saying this, it could well be true. It might be worthwhile to speak with your vet and arrange to have your puppy checked. Most vets do know who is producing a lot of puppies on a regular basis in their areas. They also know the overall health status of the dogs in these breeding kennels. If your puppy is healthy and has no problem the vet can detect, and you only want the puppy as a pet, then it won't matter where you got it from.
However you would know in future not to go and buy from those kennels again. When you saw the mother was she in with the puppies? That is usually a good indicator that the bitch you have been shown is the puppies mother, as you would see her tolerate the puppies better than a strange bitch.