If you are searching for a particular seed, for example, you can go to the gardener's exchange . At this location, you can first search all previous postings to see if anyone has offered the seed you are looking for, and then correspond with them via e-mail. You can describe the seeds you have to offer in trade, and the seeds you are looking for. Regular readers who have that seed will respond. This is the place to search for hard to find seeds.
Gardeners are the most helpful people in the world.
From time to time, you might get a request for seeds from someone who has no seeds to trade. You can ask for postage in advance, or you can be generous, depending upon available seeds, packing materials, and your ability to absorb the expense. For a fair trade, offer seeds which you have collected and cleaned from only the most recent growing season. The number of varieties or the number of seeds per variety traded are of less consequence than equalizing the postage expense of each party to the trade. A small postal scale will quickly identify shipments requiring more than one postage stamp.