The Whistle Post has a classified section.http://www.thewhistlepost.com/forums/announcements/8080-check-out-classifieds-buy-sell-trade-auction.html
There is an HO scale yardsale group on Yahoo.
There's this one as well.
http://www.modelrailroadforums.com
Chip
Building the Rock Ridge Railroad with the slowest construction crew west of the Pecos.
You can try this place. www.railroad-line.com
You will have to register and join the site. Down the page under Company Store is a forum called Swap Meet. I have never used this site to exchange anything.
It takes an iron man to play with a toy iron horse.
Only exchange ideas and experience - THAT should be enough
are you allowed to trade here