Its easy, just start a new discussion for the item you want to list. Don't forget to include either the price or what you would like to trade it for. The easiest way to start a discussion while in the Forums is to click "+Add" at the top right of your screen.
Rather than only being a Classifieds, it would be fitting with the bushcraft spirit for us to trade natural resources we have access to. If you have plenty of flint in your area, someone in another part of NZ might have access to plenty of fatwood and be willing to trade.
Make use of it, that's what a community is for!