Seed exchange is typical of annuals. How about tree cuttings/ seed exchange? It shouldn't be too hard to mail these to each other in exchange for shipping and handling and/or a trade and/or a small profit. Does this already exist?
If you want to participate list the species/variety and the locale where the tree has grown successfully (plus any advice you have). Include your terms. Also include what you currently need!
Example: I'll soon be planting two apple tree varieties: Liberty and William's Pride in USDA zone 7b, North Carolina. I'm also planting a Celeste fig. I could offer cuttings for trade when it's time for pruning.
I'd like to have some white willow cuttings for building a live fence. I'd like additional apple varieties for grafting. I'd like persimmon and paw paw seeds.
I love seed/cutting/grafting/budding material exchange.
There are a few well established communities that do this already, but that's not saying that there is enough of them. I don't know of any that focus on permaculture plants, although many of them include a permie category.
As much as I would love to participate, I'm not in the US, and sending seeds south across the 49th right now seems to end in the seeds being destroyed - something to do with new processing machinery, they say.
One concern about sending living stock/cuttings/roots/ &C. is that there are some quarantines set up about moving it across different parts of the country. At least that's how it is in Canada, although it may not be that way in the US. The biggest concern is plant diseases being spread. You would need to find out for yourself what the restrictions are for your part of the world and if you want to take the risk of breaking them.
For example, we live in a potato quarantine zone, where there are these nasty nematodes in the soil. One of two places in Canada with them and apparently we've had them here since the 1950s. The quarantine zone is actually quite small, but it's ensured that there has been no spread of these nasty potato munchers since their accidental import (because some buisness man ignored the quarantine that was already in place and imported special potatoes from a far off land that is known to have these nematodes). Even though we don't personally have the nematodes because we are on the edge of the quarantine zone and not in the infestation zone, taking the potatoes for a two minute drive would still break quarantine. And maybe it would cause risk, maybe we have them but haven't noticed because we alway harvest our potatoes young for new potatoes and not as storage potatoes.
Another example of a problem we've had happened when someone gave us some grafting branches for our apple tree. We didn't know this at the time, but they have this fungus that makes big nasty splits in the tree. My grandfather grafted the apple branches onto the root stock, and being 95 years old and knowing better about these things, he did not disinfect his equipment or quarantine the new trees. Now we have over 200 fruit trees with this infection and it's probably in the soil as well, so anything we plant here will get it, no matter how careful we are with disinfecting our tools. Worse, if we sent branches, it would spread this to someone else. I don't know if the first person didn't recognize the infection, or just didn't care if he gave it to us, but we have it now and we won't spread it.
I guess, what I'm saying is that sending live material has a lot more risk than seeds. You have to trust the other person to be knowledgeable and honourable. If there is anywhere on the internet with people both knowledgeable and honourable, this is the place to look.
Most of the people here already know these resources, but I thought to mention them for those readers new to seed exchanges.
In the USA (which includes world wide members) there is the Seed Savers Exchange which includes a shop, but focuses mostly on person to person exchange of seeds. They publish a catalogue each year, listing what varieties people have available and how to get in touch with the person. You send off a request with some money to cover shipping, and in theory your seeds arrive soon.
In Canada there is Seeds of Diversity which is very similar except no store. I don't know if Seed Savers Exchange does this, but Seeds of Diversity includes some rootstock, some perennial seeds and budding/cutting material. Some people even except Canadian Tire money - woot!
Both of those require membership fee to cover the publication and infrastructure costs. It's a moderate fee, but when you realize that the varieties available are not accessable anywhere else, it's well worth it if you can afford to join.