DaveMage
Slumbering in Tsar
"Fully versed" is a pretty high bar for DMs.
I buy a published setting because something about it gets my creative juices flowing. I buy it because any time my players say, "We head south. What do we find?" I can look at a map and answer them.
I don't buy it to spend hours memorizing trivia so I will know what secret door is hidden in what inn, and what the name of every blacksmith/bar maid is.
And that's cool - as long as you're not DMing fans of the setting who want a DM with more knowledge of the setting.
But part of the appeal of the Realms to some people (not you apparently) is the fact that it *is* so detailed, and in their imagination, they've practically lived there. It's likely to be less fun for them if you don't know the setting as imtimately.
Of course, if the players are willing you can always compromise. If all you are familiar with is the campaign setting book only, then simply let them know that if something is not in the campaign setting book, then it's just a rumor. (Maybe that secret door they think they know about is actually a trap to lure suckers [a.k.a. adventurers] to their doom - probably set up by the local thieves guild.)