Solid idea,
A system wouldn't even be necessary, just an extra category in the marketplace section where people can start threads for themselves where people they deal with can post feedback. I.e. thread title: "Feedback for lucidity", etc. Having the person in question create their own thread would keep things uncluttered and easily searchable.
Its not foolproof. But its about the easiest and cheapest way possible to go about a feedback system. Of course considering how 9 in 10 people are incapable of using the search feature allready the mods probably would be left condensing a lot of feedback since most people won't read the instructions for the section.