I tell people the same thing every time: a good query letter (including a synopsis and/or outline) isn't meant to make a sale. It's meant to inspire the editor and/or agent to request more of your writing. That's it. The query letter should get you to the "Can I see the first few pages" or outline or whatever the editor's preference is for a next step. That's what you want - because working with an editor isn't a DEAL, it's a RELATIONSHIP. And the query letter is the first step on building that relationship. You editor will be your book's champion, and your best advocate. And you have to start that somewhere.
So. You don't really want to make a sale with just the query letter; you want to start what you hope will become an excellent and fulfilling professional relationship.
As an addendum: the difficulty of doing this is not to be underestimated. Sometimes you get lucky, sometimes it takes LOTS of queries and LOTS of false starts to find that editor. And you often have to do that with each new project, unless you do well enough with one book that the editor is already interested in whatever's next. This is one reason I planned the IMAGINARIUM GEOGRAPHICA books as a series - I'd always know what was coming up, and my editor would be primed for more books from the onset. No extra queries to mess with. This is where the 'relationship' part comes in.
When my publisher offered me the contract for books 4 and 5, I was a little nervous - I hadn't even delivered Book 2 yet. I voiced this to my manager, asking, "What if we sign this, and the second or third books tank? What happens then?"
She said she asked my editor the same thing, and his response was, "If that happens we'll just convert the contract to do two other books. We just want to be in business with James."
Like I said - you aren't just trying to sell a book. You're trying to build a relationship. And it starts with that first query and outline.