Dave has a great tutorial on using the swap tool now... easier than going through the instructions I've posted previously (I write lots of little steps 'cause I don't want to miss anything -- but that makes for a long list that can look intimidating before you try it).
If you look at the list for the swap, you'll get the idea of how we're using the fields for the seed swap for plants... instead of number of seeds per packet, describe the plant (tree in 2 gallon pot, seedling not hardened off, etc), that sort of thing.
It's pretty intuitive, and Dave has tweaked it to be more user-friendly yet as we've gone along. :-)
If you enter things now, people can check out what you think you'll have, chat here about them, etc. You can also look at what other people have on the list and mark items with stars to put them on your "wish list" before trading starts... that helps you know just where to "spend" those first few dibs!
