How do you gain credibility - what makes you credible?
The best way to gain credibility is through long experience with a person. Over time, you come to trust that she will carry out what she is supposed to do, and you are willing to trust them with important items.
But if you are a startup - none of your clients or early customers have any experience with you. So how to create credibility in that situation?
Let's use an analogy: a job interview. How do you know that the person you are interviewing knows the job? How do you decide on the credibility of a candidate? If the candidate seems to know the industry, ask useful questions, can explain how things are done - in this and a myriad of ways, she demonstrates competence in the role.
Likewise, you gain credibility in your prospective customer's eye, by doing the same: asking good questions, show understanding of the industry - pay attention to the issues they are interested in. These are ways you can bank 'knowledge' credibility (credibility based on what you know).
Another way to base credibility is in 'who you know'. If Jones always trusts you to do the job, and I trust Jones, then there is a transitive relationship. This 'transitive' credibility is harder to build, since presumably there are not other customers you can refer to. But there are people who know the industry and are interested in your work. Be sure they can help you.
I will leave you to work out how you can lose credibility - but suffice to say, both lack of knowledge and lack of transitiveness cause it.
As you build your business: are you building credibility?