Some General Guidelines . . .
Most reputable breeders will only sell a dog under contract, which will vary from breeder to breeder, but which will set forth that breeder's policy regarding health guarantee, refund/return policy and other rights/responsibilities between buyer and seller.
A reputable breeder will offer you support with your new puppy, and always help you place the dog (or take it back) if for some reason you cannot keep the dog.
A reputable breeder will be knowledgeable about the breed and the genetic diseases which are common in that breed.
A reputable breeder will carefully screen potential buyers to ensure that the dogs will be placed in an appropriate home.
A reputable breeder is willing to answer your questions about the breed and the appropriate care for your dog.
A reputable breeder will require that your puppy be spayed/neutered at the appropriate age.
A reputable breeder will be able to provide you with refrences from previous puppy buyers and their vet. DO NOT rely on internet review sites for choosing your breeder! There are many good, and bad reviews listed on sites, the only way to really know for sure is to actually TALK to the breeder, ask for references and actually take the time to CHECK THEM OUT PERSONALLY!!!
Is the breeder an AKC BREEDER OF MERIT? This designation from the AKC is not given out to just any breeder. Breeders must pass a physical records and health check from an AKC REPRESENTITIVE!!!
Ask about the health of the other dogs in the home.
Ask to see photos of the puppies... Puppies should be nicely rounded in shape, without being over fat or too thin. Most healthy pups will have shiny, alert eyes and healthy looking coats. Ask the breeder if the puppies have been tested for worms, dewormed and vaccinated.
It is common for breeders to use a stud dog that they do not own, but they may have photos of the dog. Quite often, the bitch will not look in very good condition after whelping a litter. The reason for this is that feeding a litter places huge demands on the bitch, so she may be a little thin, but not skinny, and her coat may be shedding due to the hormonal changes caused by pregnancy and milk production. She should however, still appear happy, lively, confident, interact well with the breeder and show no signs of illness.
Ask the breeder if the parents of the puppies have had all recommended health clearances. Border Terriers should have an OFA Eye, Cardiac, Patella and Hip Exam (after age 2). You can check for these clearances on the OFA website at Orthopedic Foundation For Animals
REMEMBER: It is ultimately your responsibility to research the individual breeder that you finally purchase from and while it is your right to ask as many questions as possible, it is also the breeders right (and responsibility) to ask you questions as well, so don't be offended if the breeder gives you the third degree!