Here's what I've just done. I've looked at Cavs - a popular breed with puppy farmers and the general public, and sadly also one with very well known and serious health issues. I've just looked at the first two pages on Pets4Homes for this breed. Of the first seventeen ads I saw (for some reason one ad was for a poodle) - there were:- pups - six Cavs, two Cavapoos, five Cavochons, one cav/springer, an adult Cav being sold, and three stud dogs.
Only two of the eighteen mentioned BVA/KC health checks for eyes, heart etc. One stud ad mentioned eye testing but gave no mention of syringomelia or heart. One stud ad was adamant in wanting cash only (hmmm, that sounds reputable then).
The sad thing was googling each of the advertised phone numbers. Time after time, more ads would come up either for other cav crosses, or other small breeds such as yorkies. The saddest was one advertiser who were also getting rid of a seven year old bitch on gumtree - nice way to reward her after making all that money off her.
Of course, all of them had lovely pics (clearly many stock photos), and all offered the obligatory puppy pack and first vax. None mentioned if they were registered breeders, and none claimed to be members of breed club.
I'm sure this snapshot could be repeated for many small popular breeds. On the basis of the absence of health checks info, these sellers are falling at the first hurdle - that of selling healthy dogs. The stud ads were dreadful - clearly only motivated by cash with no care for the serious, heriditable diseases they could be spreading around.
What I saw tonight has not persuaded me - in fact quite the reverse. Buying via this website is not a responsible way to source a dog.