We've come to a decision on how to proceed.
We're going to have the coder who created our editorial system complete the classifieds addon.
Because
there is already a classifieds addon in the resource manager which makes the market for a competing product smaller (this wasn't the case when I started this thread)
it will be more expensive to code the addon to include features which other admins will demand, but which we don't need and
the market value of addons at Xenforo is so low that the realistic price for our addon (in the $hundreds) would be considered by most folks to be too expensive
we have decided to not release our classifieds addon for sale on Xenforo and keep it as a bespoke solution on AVForums.
Many thanks for your input and support and apologies to anyone who is disappointed by this decision.