Here's how our classifieds work:
Our goal is to include every 150-152 for sale. We get some ads from members, and some from FBO bulletin boards etc. The majority of the ads are originally found on other websites. If we find an airplane for sale on Trade a Plane, ASO, Aerotrader, Controller etc. we email or call the seller and offer to advertise the airplane for free in our classifieds. As you might expect, most accept.
For several months a couple of years ago we actually had a full time employee (Terri Koen) who did nothing but contact sellers and update the classifieds. While Terri was on staff we had an average of 200 airplanes listed. Unfortunately, the classifieds don't generate any direct revenue, so when Terri left for Dental school we couldn't afford to replace her. Since then we actively "fish" for new classifieds every 60 days or so, updating the ads just before we send the newsletter to the printer.
Because we want to include as many airplanes as possible as a service to our members who are shopping for an airplane, we don't charge for the ads, but this means a lot of sellers never bother to inform us when the airplane sells. To keep our ads from going stale we set our system to automatically expire ads 60 days after they are placed. Ideally, this means the ads are freshened and confirmed every 60 days in combination with the newsletter publishing schedule.
Because of the tremendous amount of time we put into the Clinton Fly-in each July, it takes us longer than usual to get to the newsletter and classified updates for July/August. The reason you see so many airplanes listed currently is that we did 3 full work days of classified "fishing" over labor day weekend.
We hope to eventually run the classified updates on a more frequent schedule. I have a number of improvements in mind, things like automated value appraisals, links to the history of the airplane, and a better gallery of classified photos.