
My experience is different, and perhaps it depends on the breed.
When I bred my first litter of only four, I kept one, one was booked by a lady who is now a good friend, and will be having a third puppy from me. I had two left to find homes for, and three people messed me about and let me down over these two puppies. I had lots of enquiries from other people, who I blithely passed on to other people who had pups, or maybe only expecting.
Since then I take a £50 deposit, if the people want to be certain of having a pup, that way I can pass on surplus enquiries, and if I am then let down have something to defray the added cost of re advertising, and waiting for new homes to become available.
My breed is numerically small, so the supply and demand for puppies is fairly balanced, and there is either feast or famine, sometimes pups have to wait for homes, as there is a surfeit of one sex say, and at other times people have to wait for a pup.