Well both my dogs I've gotten off gumtree...awkward moment when both of them are awesome and healthy. I never saw Bonnie's parents with another litter, try though I might before we got Rebel (plan was to desex her before the male came home if it was from her litter so no surprises). I've actually got tons of pets off gumtree and they've all been great healthwise - don't know where you guys live so might be different there. I've had some trouble with buying birds off it occassionally but that's to be expected.
As far as the RSPCA goes, well people turn to gumtree because they're so expensive. Something like $400 for an adult x-bred, a lot of the adults are not even that attractive looking (contest me on this but I like to have a good looking dog - temperament and health come first, but first impressions also count - say what you like, it's true - I also do not hold with the 'dogs are ornaments' argument at all). And the puppies, usually cute as hell, I think last time I checked was 500+. It depends on the type of puppy but they absolutely price them according to breed popularity which is wrong imo - all or none. However I hate the RSPCA and I think they've turned bad - I looked up a bunch of other associations and they're between 100 and 400 generally (puppies). Also, the RSPCA cuts corners - I've known a fair few people that adopted RSPCA pets only to have them catch a disease they were supposed to be vaccinated against, and one of my friends had a cat from there who, despite being labelled desexed, bore 7 healthy kittens 6 months after she was adopted! Bet the family was well pleased about that considering they lived in an apartment. But my point is, gumtree and so on are cheaper, and people go there BECAUSE the RSPCA etc charge way too much. You can get a purebred, or something very close, for the same price as a street puppy that looks like it might be a pitbull cross [NOTE- before you jump on me, this is from a general and hopefully unbiased standpoint - I don't have anything against pitties, but a lot of people do because of reported 'pitties' attacking]. (I'll be getting my next dog from a breed rescue (GSD <3) if it's not a papered breeding bitch, but otherwise I will be).
There's probably as many bad owners as there are BYB breeders - out for a quick buck (breeder) and a cheap ornament (owner). Lots of people I know with dogs who have behavioural problems like howling, all day, because nobody was home. I don't think you should generalise a whole site or type of site when there are obviously exceptions, and quite a few I'll go as far to say. The fault lies in the ones who do wrong, but there are loads of 'good' breeders on gumtree.