And now you're just speaking in hypotheticals. If there was no source material (three best-selling books), they might have a hard time selling it based on the trailers we've seen so far.
But that's not the case here, since there are three very popular, best-selling books, the property itself is able to use that to generate interest on top of the trailer. People are seeing the trailer, and while they maybe don't see as much as people who've read the books would like to see, at this point most know at least someone who've read the books and are recommending it based on that.
Point is, they don't have to rely solely on a trailer to sell this movie.