Editors and agents don’t care if a few pages are out there when you’re submitting. After you’re signed up, though, they’ll probably want the material pulled down because you’ll be revising it. You can re-post the final excerpt on your website/blog to help you promote the book when pub time comes. As for people stealing your idea pre-pub, it happens. Heck, people will steal your entire identity online, what makes you think your intellectual property is safe? I’m not a fan of posting pre-published material for that very reason. You can likely prove you had the idea first by using blog dates and the like, but why would you want to put yourself in the position to have to fight that fight?