She is a darling pup, but $550 is steep for re-homing a pet, un-spayed, no matter what the breed. What about health?
I agree that if this pup came from a reputable breeder, the breeder would be very upset to learn she is being sold on Craigslist unspayed.
I would love for you to get her so that she doesn't end up being used to breed by a bad person.
Best wishes in making your decision.