JJJ is right about how hard it is to get 300 words per page, for a 500 page novel. Assuming the average novel is 500 pages long, the average word count in story games on this site is about 46 words per page, that's a 23,000 word book. A story on this site with a nice word count is around 77 words a page on average, that's a 38,500 word book. I've been told I write long pages by people who playtest my unpublished stories. My pages average at 181 words per page, that's a 90,500 page book. That's writing all day, and struggling to make the content worth reading and not just filler text - and longer to make sure grammer and spelling is right. And I'm not going anywhere near 500 pages.
A 500 page, 300 word per page book clocks in at around 150,000 words. That's a hell of a lot of words, and on top of that, when you try to publish something, publishers and editors like to cut away sections of the book that aren't important enough, or appear to be just filler text, because the more words, the more it costs them to produce. On top of that, publishers are often hesitant to publish something by an unknown author, as it could be a risk. On top of that, they are even more hesitant to publish something from a really young author, who doesn't have much experience yet. So, this Eragon author must have been good at his job - not just a published book, a movie made of it too! I personally don't think he would have been published just because his parents owned a publishing company. Of course, you never would know.