Trades or sales aren't permitted on any of the discussion forums. That's what the classified sections are for.
However the knife classified section rules state: "No selling or trading of items you made new" That starts to step on the toes of the sponsors, though there's a fine line where someone can put up a few items for trade. When in doubt, PM the mods. If only done occassionally and without the intention of making a profit I've seen it allowed.