We could start trading threads in the "Seeds" forum.
Or wait for trading features to be added to the Plant Database.
For this season, Ella's Hog Wild Seed Swap will start up soon (I guess over in cubits). I put that link in my signature block so I could check daily).
Her annual swap has become my Holy Grail of seed swaps. I think that the Svalbord Vault got their idea from seeing her past swaps.