Twiggley,
In answer to your concern about not having something I am seeking - because I really do like trading and sharing my plants, I have offered and several times done a trade where I come up with what I feel is my "value" for what someone wants (plus the shipping cost that I would pay to get it to them) and suggest that if they find that acceptable, I would find a plant online that is worth about the same (with the cost of shipping to me) and they would order and have it sent to me. I would dig and send them their trade items and everyone gets what they want! I have received Hostas, Corydalis, even a yellow Lilac in this kind of trade in the past - in exchange for what the other party is seeking of mine.
I also once met a trader about 2 hours from my house, at a mall, while I was travelling to another state to spend the weekend with my sister!
So really, trading is only limited by your creativity and willingness to communicate what works for you while listening to what works for the other party.
Jen