A place you can post your job and hire freelance editors. With nearly 60,000 editors on the site, however, it can be a daunting project to find the right editor. To narrow down the candidates, you could make a detailed project description with required skills, language competency, and portfolio requirements. This should help eliminate unqualified editors. You can also include a random requirement like “Respond with ‘Hey, Jedi!’” in to find those detail-oriented editors who actually read your entire post and follow directions.
Even if you wrote the book in your free time, this is my day job. The per-project cost many new authors would consider “manageable” would, if broken into an hourly rate, end up paying editors less than minimum wage. They don’t intend to pay what someone could earn making french fries; they just greatly underestimate how much time must be invested in editing.
I like the idea of embedding “fine print” into the initial email. I also think it’s a good idea for an author to check references; the questions Val suggested are great. Third, as another commenter said, an editor can give the author a general assessment of their book, which will help the author decide if that particular editor will be a good fit or not.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.