This really is a case of Caveat emptor, "Let the buyer beware".
Many individuals set themselves up as a copy-editor, or proofreader, but in reality only carry out spelling, punctuation, and grammar checks. For writers who wish to self-publish, and sell their work commercially, this is not enough.
When looking for a professional editor, it is worth checking for membership of an editing body, and/or any training completed. In the UK:
Number of years' experience is a useful factor, as is an understanding of the process of writing fiction, and maybe some connection to, or understanding of, the publishing industry.
Check if the editor carries out a sample edit of your manuscript (if this is free, all the better!).
Be wary of an editor who asks for the full fee up front; some kind of deposit is acceptable.
Also be wary of an editor who's happy to rewrite large chunks of your manuscript. This is not part of a professional editor's remit; the writing belongs to the author, and should reflect their own style.
Don't expect immediate availability, and be circumspect if there is. Professional editing can take months. Saying 'yes' to a new project straight away could be a sign of an editor taking on too much work in one go (how will that affect the service?); however, every freelance editor has to start somewhere, so it may simply be that you have found one who is just starting out. Don't let this put you off.
As with any purchase of goods or services, it pays to shop around.
* Chapterhouse has earned itself somewhat of a negative reputation amongst individuals and societies; however, it is a reasonably-priced place to start for those who genuinely wish to learn about copy-editing and proofreading.