There is no such thing as an accidental litter. If you have an intact female and an intact male and the female goes into heat...you will have puppies unless you are one of those extremely rare types who is very diligent.
Secondly, 6 weeks old is too young to be away from the mother and litter mates. I would never want a puppy that was taken away too early as it can lead to problems down the line. Also it is against CL (Craigslist) rules.
If she isn't trying to make any money off of them, I assume that $100 covers their neutering costs? If not, than she is SELLING them, not adopting them out and that is against CL rules.
It sounds to me like a scam and she is really only selling them for $100 because she hasn't put any money into them (shots, deworming, etc) and because she knows the economy is bad and it's better for her to get a few bucks than to have to take them to the shelter when she finds she can't sell them. Every week we are getting in entire litters of puppies because the BYBs who bred them are unable to sell them.
Plus she is a rule breaker and that is a huge pet peeve of mine.
Whenever you mix 2 breeds, you really cannot predict what the temperament or personality outcome will be. I have known both Cairns and Yorkies and have enjoyed them both. The Cairn seemed a bit yappy though and I know Yorkies can be too at times. It's kind of a gamble.
It sounds like you have already made up your mind about these puppies. I wish you a lot of luck. Google "how to find a reputable breeder" if you change your mind and want to hold out for a healthy, well bred dog. http://www.akc.org/future_dog_owner/puppy_buyers_beware.cfm
Check out some of the puppies here to see what you get when you purchase seemingly healthy pups from a breeder who does not do genetic testing, etc. ahead of time. www.stoppuppymills.com www.prisonersofgreed.com
If I was going to risk getting a pup from a BYB, I would actually prefer to get an adult from a shelter instead. Why? Because they are the same quality of pup or dog that you would get from a BYB and not only would you not be supporting the practice of ill-breeding dogs, but you would also be able to interact with the adult dog to learn of it's mature temperament as well as many health issues would have surfaced by then. When buying a pup of unknown history, you never know what kind of adult it will grow into. Once they reach maturity, their cute puppy personality can change drastically.
I hope you don't think I am being mean here, I am really just trying to answer your question and offer you some important information for you to consider.
Ask someone who is in their 30s who Corky was...if they remember the show Life Goes On.