Big ones (actually, very little) that most people miss is the little metal bits we never think about- fish hooks, needles, pins, safety pins, etc. Especially good ones.

You'd be amazed how many of them you can pack into a five gallon can, and how heavy it will be. But if things tank so badly you are bartering long term, those will be things you can't make so easily and even then they won't be so good.

Oh, and try these old threads:
http://forums.equipped.org/ubbthreads.php?ubb=showthreaded&Number=68048
and
http://forums.equipped.org/ubbthreads.php?ubb=showthreaded&Number=60273

Those are the ones that turned up in the search from "barter".