That would be fun! Unfortunately...I have no idea how to build a Craiglist-type site to offer seeds for sale or trade. I wonder if there's a way to use the peerhub.com link above to set up some type of category that would allow for searching a specific group of people offering something like seeds.
There would probably have to be some guidelines on the seeds too - for heirloom tomatoes, for instance, if you grow them in proximity to hybrids it can mess up the seed quality.