Way to go
Have you thought about having them spayed/neutered before you send them to their new homes? that's going to be your best bet in making sure they really do get altered.
I know some breeders require proof of spay/neuter from a vet before they release papers...but some people will breed anyway and register with one of the off-registries. My cat's breeder has in her contract that if the contract is violated, buyer ownes x amount of money and cat and all offspring can be reclaimed by the breeder.
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~Magnifique Yorkies~
Purchasing from backyard breeders, pet shops, and puppymills perpetuates the suffering of other dogs.
Educate yourself and buy from reputable breeders or rescue.