Welcome to the forum.
I wouldn't buy a dog from Gumtree.. sorry. What kind of papers are the people giving you?
Registrations papers or just vaccination papers?
Always go to a registered breeder, where you get to talk and meet the pups parents.
both registration and vaccination i believe, one of my friends got a JS pup from there several months ago, she got to meet the parents and stuff and said the pup was wonderful and no health problems
anyway... i guess that could have just been a one off thing... :\