Unfortunately, providing they have a licence from the local authority and they are not breaking any of the animal welfare laws, it is not illegal to sell multiple breeds.
If any laws are being broken, or they are falsely advertising the puppies (as champion sired when they are not, or with fraudulent pedigrees for instance), you can report them to the local Trading Standards.
We have a local puppy farmer specialising in 'our' breed and even though the puppies are produced on a large scale and often sold without a KC registration (this is only provided if the purchaser insists and keeps asking repeatedley), they are not breaking any laws, so cannot be stopped.
They have a permanent advert in the local paper and are a very good customer for the advertising department, so the newspaper will certainly not ban them!
snomaes