Why not to buy from a newspaper ad or pet shop
When purchasing a Cavalier (or any other breed of dog), you want to avoid pet shops, puppy mills and backyard breeders. Pet shops are easy to pick out. Puppy mills aren't so easy to recognize. This website has a lot of information on how you can avoid a badly bred dog and why you shouldn't buy from any of these places.
Here are a few other websites with important information:
In general you never want to buy from anyone who offers to meet you with the puppy 'for convenience' and avoids having you go to their place to meet the mother (and ideally, the father as well) of the pups. There are likely brokers selling puppy mill/puppy farm dogs.
Investigate the going price for good quality cavaliers in your region. Typically backyard breeders and brokers of puppy mill dogs will be just a bit cheaper to make their dogs more attractive. Alarm bells should ring if they tell you they have a relative or friend in Ireland who breeds 'Irish championship stock' puppies. Almost certainly these are puppy mill/puppy farm dogs being sold ruthlessly through a broker. About 25% of such dogs die in transit, packed in crates in airplanes. Papers from Ireland are often forgeries.
In any case papers are not necessarily a guarantee of quality, as bad breeders can get their dogs registered too. A dog in the US that is being sold with any other American registration than AKC or CKCSC is probably a backyard bred or puppy mill bred dog, as others are worthless registries that anyone can use. Millers regularly use them.
If you are told you don't need papers because you are getting a pet, not a show dog, do not take the puppy. Good breeders register litters after birth and the papers are a sign that the dog's breeder should be complying with the ethics of his or her breed club and that the dog comes from other guaranteed purebred dogs -- this is at least some step in insuring a quality dog.