I agree with being allowed to mention you have something for sale, but I don't think selling directly on a forum is a good idea. As often seen on Yahoo Groups, a bad sale turns into name calling and threats many times. This often creates "factions" and wrecks the entire group.
Since "common sense" is becoming so hard to find in today's society, let alone on the Internet where you can say and do things you wouldn't DARE to do face to face, I'd avoid sales.
There are plenty of websites to buy and sell trains, and I think there's a new website to buy and sell On30 stuff specifically being talked about on both the On30 and On30 Conspiracy Yahoo groups right now.