Sheltie isn't a mixed dog - it's a breed.
Reputable is one that welcomes and encourages inspection of the property and all breeding stock, and whatever puppies are still on the property.
Any furtiveness, evasiveness and excuses should get your antennae up and have no more to do with them in that case.
When you have decided on a breed, ask your local vet if they know and can recommend a breeder.