What is one thing I can do to make my book more marketable to publishers?
I am not kidding about this: mechanics, mechanics, mechanics! If you’re not sure how to do something format or grammar-wise, pull out a book by the author you some day want to outsell and do it the way s/he does it.
Flawed grammar and sloppy formatting are the two biggest turnoffs for me when I read fanfic. For example, take a good look at every single book in English published since almost the dawn of time: the comma goes inside the quotation marks.
If you would like your book treated professionally, then you need to present it professionally. That doesn’t guarantee that you’ll receive professionalism in return, but don’t give anyone an excuse to set your manuscript aside.
Should I pay a potential publisher a reading fee?
No, absolutely not! I do not know of a reputable publisher who will charge you to read your book. Investing the time to read it is their risk in the process. Your job with your cover letter and synopsis is to make them want to read it. If you are looking for a professional evaluation of your manuscript before you send it to a publisher, then find a professional and pay for it.