Actually, I know REPUTABLE BREEDERS who do advertise. And still screen homes carefully and all that jazz.If those regular families gentically health test their dogs, and their dogs have excellent temperaments...that would be decent. But reputable breeders don't advertise, so you probably won't find that.
You would be buying from a BYB, which usually don't breed for temperament or health, so you could end up with a messed up dog, it does happen.
Reputable breeders, you have a pretty much almost perfect chance of having a very healthy, and tempermentally sound dog. Although sometimes problems occur, but they are few if the breeder knows that they are doing, and has good breeding stock.
*shrugs* it can and is done.