There are a few breeders who prefer not to sell any of their kittens as breeders but for the most part, at least with most of the breeders I know, the decision is based more in line with what Deborah described, deciding by comparing a kitten to the written standard, or if the kitten has a nonstandard coat or pattern it will most likely be sold as a pet. I personally would much rather have my kittens all go as pets, but if there is one who I think will excel as a breeder or a show kitten I would rather see it used in that capacity as it will serve to improve the breed, which is one of our goals as breeders (and we can't keep them all ourselves!)