go to the flyer forum, they have a "classifieds" forum on there and you can put your ad up.
Good suggestion but what if the person who wants something similar to what the person is selling is using PPRuNe, and he doesn't visit the Flyer forum?
Whats wrong with the classified, any person with an ounce of common sense would ask to view the aircraft before they buy. Sure it may cost you a trip but may save you tens of thousands by just simply inspecting the plane. You go to by a PA-28, you get a C-150, buy a twin, get a single I understand that it's of concern but go inspect the darn thing.
If Flyer can do it then why not PPRuNe? (I see a few 'mod material' people here so the lack of mods is fixable)