Matt- of course there are slight differences, but auctions and direct sales could easily have their own sections. Perhaps the auctions could be placed in the section for ebay and catalog auction announcements.
With regard to straight sales getting bumped down, there is no harm in a seller simply bumping his own thread when he feels it fallen too far down the column. That in turn could bring about a bumping war, with sellers and auctioneers fighting for the top spot.
