Expertise is relative to your audience. If you (at least pretend to) know anything more than your prospect about something, you'll be perceived as an expert, which is why "gurus" and the like are able to sell WSOs, including those containing drivel like "the money is in the list," "build trust" and "deliver value."
Being perceived as an expert, or any sort of personality marketing, is really only needed if you're selling something with a limited value proposition. To sell something that actually has a value proposition, you only need to know enough about it to sell it, specifically to explain the value proposition and answer questions about it.
The real money is in a customer list.