yes i agree if did stud my dog would put him with somebodys bitch i now and trust or one of my own first time.
but if the bitch got pregnant then would want payment because he his then proven but if no pups would not take any money

I think there is also something that needs to be thought about.
Although you do not own the bitch or puppies born, it is your name/ breeding behind the stud dog. So I think if you did not know the owners of bitch very well as in their Ethics etc... you should not be studding your dog with them as personally I would feel terrible if the puppies born were sold to whoever. So basically I think you need to know as much about the owner of bitch before they use your stud as he is a big part of the picture

So does offering a "free until proven" attract those purely interested in a free mating,not quality or bloodlines?

IMO Free Mating. What I would like to ask is why do people who own a Stud want to advertise a "Free until proven" if you are meaning that this would be an advert of course