Transcribed from Stephanie's response to a post inside a group for writers helping writers answering this exact question.
Speaking from someone who has spent a decade cultivating her marketing skills specifically to be better at marketing herself and her businesses there's a few reasons I wouldn't consider this:
1) Product Quality
The people who are usually asking for this....don't usually have a great product for me to market and marketing is only as strong as the product/company it represents. There's a LOT of things that could sabotage the results here...If the book isn't great... if the interior isn't great, or the cover isn't great, or it's set up incorrectly for searching, or the author's personal brand ends up being damaged or negative, or the author doesn't want to be involved or doesn't have a marketable personality...
That's a LOT of potential problems, some of which might not be discovered until a lot of work has already been put in.
2) Skills Are Worth Paying For
I spent a lot of time cultivating my marketing muscle...I'm good at it. I'd rather use it to promote myself if I'm going to do it without getting paid up front because then I at least control all the variables on the back end.
Finding a good marketer and asking them to do it for free is the equivalent of asking an artist to do it "for exposure"... except, those of us who are good have people lining up to pay us because it's an insanely valuable skill in the business world...why would we take on all the risk listed above to do a bunch of work for free with all the potential variables at play...? It just doesn't add up....
To an extent, you could look at this as "If your manuscript isn't good enough to get picked up by a professional publisher, who would do this...what guarantees do I have that it's going to be good enough to back up the marketing I design to promote it?"
And I know that's not 100% accurate all the time...but it's solid food for thought!
That also said - I get the scammer frustration with marketers.
A lot of marketers are better at marketing themselves than anyone else because they don't understand marketing...they're just mimicking what they've seen other people do so they can appear like they know what they are doing.
I spoke with someone the other day who had built one website for the business she worked for as a secretary...and then wanted to start a marketing consultancy...I was just trying so hard not to groan.
But that's COMMON.
If you're looking to hire a marketer, do NOT be afraid to ask questions!
I told someone recently, if they make you feel stupid for asking questions, it's because they don't know the answer and you don't want to work with them.
So ask the stupid questions.
Ask for references (and call them).
And know that you're going to have to put in time to market your own book too...even if you get a great marketer on your side!