This is one of the first verticals I'd like to target (if there's a need).
What do you use to take payment & fulfill your orders? What would get you to switch? Is getting exposure from a marketplace more important than a low transaction rate?
Reminds me of Jesse Storimer http://jstorimer.com/ who originally published 'Working With Unix Processes' independently which subsequently got picked up by The Pragmatic Programmers: http://pragprog.com/book/jsunix/working-with-unix-processes
Currently working on some books myself.