I agree with dog is my buddy. There are bound to be black sheeps everywhere, but if we make the procedure too tough or troublesome, it might deter away a lot of genuine adopters.
So in the end, what I believe matters most is that as many dogs as possible get a chance to be rehomed. I'm sure the happy and successful ones far outweigh the unlucky ones.
In fact, if you think of it, the "unluckly" dogs aren't really that unlucky. Once they get re-sold, they will still likely get a proper home in the end, right?