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

Swapseeds.com  
   Swap seeds and house plants, also provides flower and shrub photographs. Uses a credit system, membership fee required.
   http://Swapseeds.com

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-Traders  
   Online community of people sharing, swapping and selling plants, seeds, bulbs, and cuttings.
   http://www.plant-traders.co.uk/

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