So no, I can't make your book a best-seller, and any editor who says he can is a quack. I can make your book better. I can improve its chances in the shark-ridden seas of agent-finding by making sure you don't turn off agents by doing things like using your thesaurus to find 5000 synonyms for "said" in your dialogue. But editing, the way I see it, has very little to do with the actual sales of a novel. Once reviewers and consumers have your book in their hands, good editing will ensure that your book isn't tossed into the reject pile right off the bat. But to get it there, you need your cover, your radio appearances, your local bookstore support, and everything else that goes into a good marketing effort.
I just tend to get a lot of inquiries asking whether I can magically get some book, sight unseen, into the NYT lists. The authors who ask me this question invariably disappear off the face of the earth after I respond that no, no editor can do that. I worry that these people are being suckered into paying huge amounts of money to unscrupulous "editors" who claim they possess the golden fleece of sales or something.
Honestly, if I knew the secret to best-sellerdom, I'd be churning out Harlequins like there was no tomorrow. But that would be sad. What a book really needs is magic - the magic that you as the author bring to it and no editor can ever duplicate from scratch. So go work your magic, and let a good editor help you polish the results so they're ready to present to the world.