It would depend on why the pup was returned. If it was just that the new owner couldn't cope with it or their circumstances changed that meant the pup was not going to be looked after properly then they should rehome again to the right person. If it was because there was a medical problem with the pup then no they should never have sold it in the first place and they should be responsible for its future.
The number of times I have heard people say they have a problem with a pup and the breeder has not wanted to know.