To me, it is on topic, if you phrase the question so that it is about writing. Maybe something like this:
What makes a YA series sell?
Naively we would assume that the better a book is written, the better it will sell. But obviously that is not the case, even taking into account that tastes and opinions on what is "well written" differ. Series such as Christopher Paolini's Inheritance Cycle have become bestsellers despite being widely regarded as badly written. So how do these and other YA books and series sell so well despite their apparent low quality? And what part does the writing play in their success?
Obviously marketing plays a role in Paolini's success, as does his young age, but with the last sentence you would be asking about the writing aspect of that success. Because of course the Inheritance Cycle, while maybe not a masterwork, obviously is not complete trash, and probably contains some things that appeal to its audience.
Finally, analyzing written works with a writers eye to learn something for one's own writing is certainly one of the most important parts of writing, which is why we illustrate so many of our answers with examples from existing works. I don't see why we shouldn't ask for a specific example as well.
If no one disagrees within the next few days here on meta, I'd just post a carefully phrased question (do take some time to compose it!) and see how it goes with it. I would definitely welcome that question, as it is one that I have been asking myself, too.