Online classifieds?

You mentioned the most-popular sites, but have you done a raw (search engine of your choice) search? Just put in the make, model, length, "for sale", and your town, and you may get some valid hits. Granted, you will get mostly the same sites in your initial results, but look down the list.

I listed my dad's TT on my personal site a few years ago, and someone found it there. It is not out of the question that smaller sites will appear with just what you are looking for.

Good luck!