I'm on another forum that does that and all that it ends up doing is flooding the forums with one line comments and other stuff to rack up the count to get it to post ads.
I agree that post minimums is not the way to go.
Perhaps some sort of additional info that is required stating someone who knows of you in the skydiving world prior to allowing one to use the classifieds like some sort of "vouch" system before they place or respond to their first ad. This would remove the anonymity aspect that scammers and spammers utilize.
Buyers or sellers could request a persons info be confirmed on any transaction they are unsure of as well.