In many cases, folks have no basis for judging expertise. However, they often need to make decisions.
When someone says "I know", there are three possibilities. They do know, they don't and they're lying, and they don't know but don't know that they don't know. Many people think that they can pick out liars, so they're only tricked by folks who don't know what they don't know.
When someone says that they don't know, there are also three possibilities. They actually don't know, they actually do know and are lying, and they actually do know but don't know that they know. However, there's a big "why bother" here. Lying doesn't make any sense and if they're wrong about what they know, how are they going to help you decide when to believe them?
When someone says "I know", there are three possibilities. They do know, they don't and they're lying, and they don't know but don't know that they don't know. Many people think that they can pick out liars, so they're only tricked by folks who don't know what they don't know.
When someone says that they don't know, there are also three possibilities. They actually don't know, they actually do know and are lying, and they actually do know but don't know that they know. However, there's a big "why bother" here. Lying doesn't make any sense and if they're wrong about what they know, how are they going to help you decide when to believe them?