My thinking (and anyone can please correct me if I'm wrong) but the general concept is that when you join, you look at the lists of all the other participants and pick out what you want. Then others will also be looking at your list and choosing what they want.
When the time comes, you'll be notified by me of what was picked out of your list, and you then pack up those items and mail them to me.
When I get them, I sort them all, and then send back to you the items you requested.
I don't think it'll be an exact "1 for 1" trade but more of an approximation. If you are requesting 20 different seed packets, ideally you would have had a similar amount requested from you from various other members.
Is this the right approach?