The Difference Between Agents, Publishers, and Editors
Almost all freelance editors, at one time or another, get the question: "Can you edit my book and get it published?"
The simple answer is no. The two are totally separate.
Freelance copyeditors or substantive/development editors do not publish books. They prepare your book for submission to agents and/or publishers, or for you to self-publish the book. They are experts in editing, not publishing. Freelancers rarely have the contacts, or marketing knowledge, or printing knowledge, necessary to see the book through to full publication. Some freelancers are also published authors, of course, and have good to fantastic relationships with their own editors/publishers, and perhaps know agents they could refer a writer to, but on the whole it's up to the writer to market his/her own book—which begins, usually, with finding an agent—after the writer has had the book edited.
Agents are as diversified as publishers. Know what kind of book you're writing (i.e., fiction--children's, genre or literary), or non-fiction (i.e., a biography, a math book, a cookbook, a self-help book, etc.) and then do your research to find out which agents rep that type of title. Agents make their money by taking a percentage of your advance or continuing royalties, and they can only do that if they successfully place a book in a publishing house—and they can only do that if the publishers trust the agents to hand them only titles that will be right for their houses, which makes the agents as picky as the publishers. They don't accept just anything and guess/hope that they can get the book in good enough shape to offer to a publisher—or spend their own money on copyeditors on the off chance that their "feel" for the book will pay off. Sometimes, yes they do, but on the whole, you're better off to have had the copyediting done prior to submission.
Publishers are, well, the ones who spend all the money and take all the risks. They pay for everything, in a market that's fiercely competitive, shelf space limited, and readers' subjective opinions the only ruler for success or failure.
In short, you want to obtain the best editor for your manuscript and then look for agents with a proven track record. It is not in YOUR best interest to shop for a "package deal."