Trading

Dave's Garden: Trading
Registered members can enter have and want lists for plants and seeds, can search other members' lists, and keep track of active trades. Also trade forums, for paying members only.
http://davesgarden.com/community/trading/

GardenHere
Internet community and forum for trading landscape plants. Also some articles on garden and plant tips.
http://www.gardenhere.com/

Gardenpassion
Seed trade site with searchable lists of varieties offered and sought, as well as tips on identification, seed collection, and storage.
http://www.gardenpassion.net/

Gardenweb: Exchanges & Trading
Members post their offerings and wants on the forums (various plant and seed categories), and can also maintain exchange lists in their member profile.
http://forums.gardenweb.com/forums/exchind/

Plant Internet Directory
Allows the Australian public to list plants for sale and review listings from other gardeners.
http://www.plant.id.au/

Seeds from Bulgaria
Trader with an extensive list of seeds, including fruit trees, vegetables, ornamental trees and shrubs, and flowers. Also a list of wanted seeds.
http://www.geocities.com/bg_seeds/

SeedSwappers.com
Free site for seed trading and swapping by seed swapper enthusiasts. Also has hints and tips on home gardening.
http://www.seedswappers.com/

World Plant Exchange
Offers free offers and requests for plant and seed traders worldwide.
http://www.worldplantexchange.com/