You weigh a bag of marbles on a scale that you have no information about the accuracy of, and the scale says that the bag weighs 342 grams. If you have no background information about the bag of marbles, your best estimate of the weight of the bag is 342 grams. It would be confused to say: "I should discount for the unreliability of the scale and take my best estimate to be 300 grams." For if one has no information about the scale's accuracy, one should not assume that the scale is more likely to overestimate than to underestimate by a given amount. So far so good. Now, suppose that instead of your using the scale, you give me the bag, I hold it in my hand, and say: "That feels like 340 grams." Again, your best estimate of the weight will now be 340 grams. You don't know whether I am apt to overestimate or underestimate, so it's reasonable to just go with what I said.
But now consider a different case. You have no background information about my epistemic reliability and you have no evidence regarding a proposition p, but I inform you that I have some relevant evidence and I estimate the weight of that evidence at 0.8. It seems that the same argument as before should make you estimate the weight of the evidence available to me at 0.8. But that's all the evidence available right now to either of us, so you should thus assign a credence of 0.8 to p. But the puzzle is that this is surely much too trusting. Given no information about my reliability, you would surely discount, maybe assigning a credence of 0.55 (but probably not much less). Yet, doesn't the previous argument go through? I could be overestimating the weight of the evidence. But I could also be underestimating it. By discounting the probability, you are overestimating the probability of the denial of p, and that's bad.
There is, however, a difference between the weight of evidence and the weight of marbles. The weight of marbles can be any positive real number. And if we take really seriously the claim that there is no background information about the marbles, it could be a negative number as well. So we can reasonably say that I or the scale could equally be mistaken in the upward or the downward direction. However, if we know anything about probabilities, we know that they range between 0 and 1. So my estimate of 0.8 has more possibilities of being an overestimate than of being an underestimate. It could, for instance, be too high by 0.3, with the correct estimate of the weight of my evidence being 0.5, but it couldn't be too low by 0.3 for then the correct estimate would be 1.1. We can, thus, block the puzzling argument for trust. Though that doesn't mean the conclusion of the argument is wrong.