> Trust whomever you want, on whatever basis you want, but that's fundamentally not reason or rationality, it's just an argument from authority [1]. No statement becomes more true just because someone said it, regardless of who they are.
A statement can become more likely to be true just because someone said it, depending on the nature of the statement and the person making it. For example, my brother is an oncologist. If I had some health concern about cancer, I would certainly be asking for his opinion. His opinions on that topic are not guaranteed to be correct, but as an oncologist, they are significantly more likely to be correct than that of the average person.
One problem with the frequent popular invocation of claimed "fallacies", such as "argument from authority", is many of them are only strictly speaking fallacious when used as purely deductive arguments, but real world human reasoning isn't purely deductive, it involves a great deal of induction and abduction as well. There is nothing inherently wrong with an "argument from authority" as an inductive or abductive argument.
> Knowledge is a true and justified belief. But trust is not a justification - quite the opposite. If you had a justification, you wouldn't need trust.
One can estimate the conditional probability P(proposition X is true|agent A says that X is true) based on observations of what kinds of things agent A (or other significantly similar agents) has said in the past, and how many of them turned out to be true or false. You can then use that conditional probability estimate to inform your own decision as to how much credence to give to proposition X. That is both trust and justification.