Oh yeah, there are definitely people who do this. Heck, I run AMS for a few folks in my spare time.
The question is determining what you're willing to spend on an ad manager vs. what your advertising budget is and then deciding whether or not it's worth having someone else do it for you. Usually it's only going to be cost effective (for both parties) if you're spending above a certain threshold, though. ie: if you're testing the waters with, say, a few hundred dollars, it's probably in your best interest to manage it yourself.
There are people who will willingly overcharge you to lose your money.
There's no magic bullet you can buy. No one cares about your books like you do. No cares about your business or your profitability like you do.
There is a point where hiring an admin makes sense, where you'd be better off writing, but you'll need to teach them EXACTLY what you want them to watch for and do with an existing basically functional ad set. So you'll need to be well versed in what works and doesn't with your catalog before you reach that point.
A forum community dedicated to Amazon Kindle owners, authors, and enthusiasts. Come join the discussion about publishing, audio, troubleshooting, models, styles, reviews, accessories, classifieds, and more!