I'll have to disagree with that. For one, buying an unstarted animal shouldn't lead to any negative feedback because the breeder didn't screw up. That's like saying you should only sell proven breeders because otherwise eventually you are going to sell an animal that doesn't breed.
Second, by selling an unstarted animal you don't face the trouble of having to mirror exactly how the breeder was feeding. I have had multiple bps that I've bought where there was some small difference between the breeders feeding style and mine and so I had to go back and try to trouble shoot what it was to get the animal to eat.