Freelance editors normally do not double as literary agents. Not only is it not our area of expertise, but it can actually lead to ethical quandaries. There have, unfortunately, been cases of literary agents saying that potential clients’ work “has potential” but needs to be edited (by them or their employee), for an exorbidant fee. It would also be possible for editors to give the impression that they are guaranteeing publication of a project, when that is something no one but a publisher can do. Many professional organizations forbid their members to combine these two roles, to avoid a possible conflict of interest.
The single most decisive factor should be the quality of the edit itself: Will this editor help improve your book? In order to assess this, nothing beats a sample edit (even if you have to pay for it). Testimonials and credentials (such as experience at a major NY publisher) can help you narrow your list, but you should submit the same sample to a variety of editors and compare their work side by side. Some editors will push too hard, and some won’t push hard enough; some will simply “get” your writing, while others will seem to speak another language entirely. In my experience, the editor’s résumé is far less revealing than the quality of the sample edit. In fact, the higher an editor’s pedigree, the more reluctant the editor might be to provide a sample edit. Don’t be afraid to insist: no one should expect you to invest in a car without a test drive!
It’s always sensible to follow due diligence in submitting your first draft of your book to an editor by making as fool-proof a copy as possible for you because first, it is your own creation, and secondly, a horrid draft sent your editor can end up frustrating both you and him or her. Make your own corrections to the extent you can, so that will make the job of your editor that much lighter and the end result would be a lot more effective.