Trustworthiness is something we like in other people. When we have trust in others, our relationships flourish and we can reach higher levels of camaraderie. The world becomes a better place when people are there for each other.
But the ability to trust someone does not come from merely having a need. As in this story above, the man did not even know Rabbi Salanter and yet he wanted him to slaughter an animal for his dinner. Had he known about Rabbi Salanter's credentials, he would have realized that he was the wrong person to ask. Yet, he had no clue who he was and was trusting that he would perform something for him that truly needed an expert hand (merely by his judgement of who he looked like). This kind of misplaced trust is a questionable practice. In matters such as these, trust can only be given when there has been a demonstration of competence and fact-checking.
In the case of money, the man was extremely skeptical and did not see a reason to lend Rabbi Salanter any money. While it is true that he didn't know Rabbi Salanter, in this case he could have given him a chance, knowing that he would only have asked because he was in need. At the same time, he also would not be obligated to lend to him because indeed he didn't know him and had no guarantee of repayment.
Trusting others, especially those who we just encounter, can be tricky territory. On one hand, how can we begin to trust someone who is new to our circle if we do not know them? A Russian proverb says, "Trust, but verify." Everyone needs a chance to become trusted, but that trust should never be placed blindly. If we are able to find out whether someone has a shem tov (good name), we should by all means seek that information out. But if we have no way of knowing, we have to begin somewhere. Start small, be cautious, but realize that people need a chance. In time, the measure of someone's trustworthiness will be made known.