If you've got nothing to go on but second-hand information that you can't validate yourself, then it's perfectly reasonable to use qualities like "credibility"

If you've got nothing to go on but second-hand information that you can't validate yourself, then it's perfectly reasonable to use qualities like "credibility" and "authority" as a sort of Bayesian indicator of the likelihood that the statement is a valid one. The "appeal to authority" is only a practical fallacy in a situation where it is feasible to directly evaluate the claim against reality itself.