I recently made a website that allows people to trade seeds and plants at no charge beyond postage. Essentially, you can get seeds or plants from others if you send seeds to others, using a credits system. I think it works a bit better than the current plant swap message boards because if you send a plant or seed to someone else, you get a credit which you can use to order any plant or seed from someone else. There is a feedback section which allows you to rate the items you ordered, and a way to communicate with other users.
In addition, you get a credit when you register, at no cost, which you can use to order one item.