I think this is a moot discussion about Gumtree and the like. Considering anyone can advertise on these sites and there are no parameters to adveritising (only BYB etc) the saying getting a dog off these places is a bad idea is not true.
It's too much to be regulated really for no real outcome. Liars will be liars - there are plenty of people who don't tell the puppies breeder they want to breed and purchase under false pretenses etc. It's up to the breeders to try as hard as they can and there is always a division on what is a base level of care for an animal. To the law food, water, shelter is enough. To many others it's a lot higher.A note on your test theory - I think if the breeder has to sit the test, then anyone wanting to buy an animal has to sit one too. It's not the breeder's fault if the buyer is a shallow, impulsive pr*ck, and vice versa. Maybe it could be based on an experience thing - a test for the first animal you buy, then you can buy freely. Maybe an example of where you see yourself in 15 years' time etc. I don't know, probably will never come to pass though.