Note, however, that an agent often wants to market the book from scratch, and has a slight preference toward a book that has yet to be rejected. These are essential to selling a novel. The sample chapters are always the first chapters of the book. An outline runs around 20 pages and describes the action in the book. Include any particularly important lines of dialog and all important scenes as you lay out the plot and all subplots.
Some agents prefer a synopsis to an outline.
Needless to say, either your outline or your synopsis must be well written and interesting. There's no reason you can't market your novel yourself. There are a few hurdles, but it's eiminently possible. It isn't the end of your career, so just keep on plugging. Sooner or later, it will click for you.