For exchange purposes, you may want to repost next spring or early summer when the seeds are maturing on the plants. The plants you want are mostly spring ephemerals which must have their seed collected as soon as mature and sown right way. If stored, even for only a few weeks during mailing, their seeds must be stored moist. So your donors should package the fresh seed in moist vermiculite in a baggie.

These seeds also have complex germination requirements. Some must be stored moist and warm for a few months, then stored moist and cold for a few months, then switched back to warm before germinating. Several have double germination--the first year they germinate and only grow roots then after a dormant cold winter they grow leaves the second spring.

The New England Wildflower Society often has some of these seeds in their seed sale in January. They also have a great book called "Growing and Propogating Wildflowers" by William Cullina which describes the collection, storage and germination needs of wildflowers.

Here is a link that might be useful: New England Wildflower Society