This would breach most breeder contracts, yes? Do breeders ever approve of online listings as a way to rehome dogs?
Depends on the contract. Most ask for "first right of refusal" which means if someone intends to sell the dog, they must offer the dog back to the breeder first at their price. It's unrealistic to think that breeders require a person to GIVE a dog back, but it's absolutely ethical for a breeder to have first right of refusal (and use it).
Some people really just have no idea that many breeders WANT to help, if not get dogs back at their own expense. They think they are doing the right thing by NOT burdening the breeder.