Being a know-it-all is a great way to make people question your common sense.
Many salespeople and most managers think that they'll lose credibility if they admit ignorance, especially about something about which they "ought" to know. However, the exact opposite is the case.
Admitting ignorance makes everything else you say more credible. Admitting ignorance marks you as a person who's not afraid to speak the truth, even when that truth might reflect poorly on you.
Needless to say, the "I don't know" should be followed by a plan to discover the information that's required, if the issue is truly important. And you WILL be judged on whether you deliver on that promise.
But here's the thing: people dislike a know-it-all. They can often sense, at a gut level, when they're being BSed. Even if they're taken in, when they find out (as usually happens) that they've been BSed, they never trust the BSer again.