Lately it seems like a lot of people are hanging out a shingle on their writing websites, advertising editorial services and manuscript critiques for a fee. As somebody who's worked professionally as an editor for 13 years (ugh, I'm getting old), this makes me a little nervous. I'm not saying that many of these people aren't qualified to provide this type of service, but I do think writers should tread carefully.
Some questions to consider:
1. In terms of big-picture critiques, is this something you couldn't get from a beta reader? There are a ton of writing sites out there (like Absolute Write, Book Country, etc.) where you can post work for critique, meet other writers, and interact. If you meet somebody whose work you admire, you can try striking up a friendship with them, offering to critique their work, and asking them to look at yours. If you get a couple of solid beta readers, you'll get good feedback from talented writers for free. Plus, swapping manuscripts is super beneficial, in that you'll learn a ton from reading and critiquing someone else's work.
2. In terms of line edits, what is the advantage of hiring a non-professional over a professional? Other than price, I guess, and I'm not even sure there's a huge difference there. You can post an ad for a proofreader/copyeditor on Writer.ly or Craigslist, compare candidates' pricing offers, and most importantly, select the person with the best credentials. There are a ton of freelance copyeditors and proofreaders out there who make their living doing this, who have years of experience and solid credentials.
Which brings me to the next point: Make sure you check somebody's credentials before forking over money to edit your precious manuscript. Check what kinds of work they've done in the past. Maybe they used to work in the editing field -- if so, awesome!
I can't stress this enough: writing a book and editing a book are two completely different skills. If you're going to pay somebody to edit your book, make sure they know how to edit. Ask for references, and if possible, get a trial or sample of their editing before you commit to a full manuscript review, to make sure you're comfortable with their style and level of feedback.
This is the part in the publishing process where you hold all the control. If you sell your book, it's out of your hands, but for now, make sure you do your homework and choose the right people to work with. Pick smart people with experience and enthusiasm and professionalism. Take your time. Don't sell yourself or your book short. You owe it to that manuscript you spent so much time writing!