So, you've got to be fair. If someone tells you something, try to corroborate the information. No one can fault you if what you say is true. Your first responsibility is to your readers. It doesn't matter how good the newsletter is if no one reads it.
Newsletters sell by word of mouth.
Of course, it helps to have a best selling book related to the letter, and many letters are tied to books, but short of that, the best way to build a reputation is to get people you write about to read your newsletter. A proportion of the circulation of any periodical is sent gratis to people in the same field. With luck, they'll mention it to their peers and people they influence. A web site can also attract subscribers while adding value to your newsletter. You can offer samples, sell subscriptions, and reprint articles.