Be very careful about puppies for sale that have both parents at the same home, chances are they are just mating two 'pet' dogs irrespective of their health, age or maybe even the fact that they may be related (mother x son, brother x sister). Also they could be mating the bitch at every season.
At least with licensed breeders and pedigree registration you know that they are only breeding a certain number of puppies from each bitch and that there is a better chance that the puppies are looked after in suitable conditions.
Most free ads I would be very wary of as well, if they cannot be bothered to pay out money advertising to get good homes it does not say much about the breeder, there is often no back up with these sort of ads, once the pup has been sold that is it as far as they are concerned.
Many puppies bought this way are bred elsewhere (eg puppy farms) and 'sold' from so called private homes. There are so many people out to scam with the free ads you have to be extremely careful that for instance the dog or puppy you buy has not been stolen.
You will probably end up paying hundreds of pounds more for a poorly bred, sick puppy that will not only cost more in the first place but will also cost you a fortune in vets bills.
You would be better by far going to a registered breeder (who maybe breeds only occasionally and keeps their dogs in the home as pets), explain that you want the dog as a 'pet' (not for show). The link lankeela gave is a good one.