I do most of my 2nd hand buying on OfferUp. Idk where it came from, a friend told me about it and it's pretty active.
airplanes, airplane stuff, airports, airport stuff, little airplanes, big airplanes, fighter jets, airplanes from the dawn times, every budget and skill level. online since day 2, almost exactly unchanged, certain adds cost a few bucks to list, other adds are hand coded into the pages, social side is listings for "fly in's"