The Mattole area (that I live several hundred miles away from) came up with their own currency in the form of silver coins they had minted, and that's all well and good, but LETS is different. The system that I referred to doesn't use a currency that has any value in an of itself, unlike the silver in a coin. I've seen a couple of other privately minted currencies and they're pretty and valuable I guess...but at the end of the day all they are is a lump of silver.
I'm attracted to the idea of a "currency" where the value backing it is tangible goods and services. This sort of a system discourages hording of resources, and encourages the re-circulation of resources within a community. Seems that it could have the power to really draw people together, if it was organized correctly.
The only "value" this sort of non-currency has IS the goods and services for which it is exchanged. That's the beauty.
AND! I just read about a man who is battling counterfitting charges for a silver currency he made and sold.
Not the article I read but about the dude:
http://en.wikipedia.org/wiki/Bernard_von_NotHaus
No such thing could possibly happen if the "currency" is a symbol for goods and services and has no value in and of itself.
There are many successful examples of non-currency trade systems. The one I witnessed personally was in the Golden Bay of New Zealand, called H.A.N.D.S. - How About a Non-Dollar System. One person was responsible for keeping track of members, it seemed to me, (also seems a weakness in the system though theirs was small scale). They even made (quite beautiful) paper HANDS "non-dollars" for a HANDS market. No real currency was exchanged at the market, only these HANDS dollars, and it was quite lively! I was invited on a sailing trip with two people who paid for it with these HANDS dollars that they acquired in exchange for food at that HANDS market.
So instead of the "value" of the boat trip being quantified in a certain weight of a precious metal (as national currencies are), the value of the boat trip was represented by food made from local ingredients, by the labor of caring people who made it, and the appreciation of the people who exchanged a paper symbol of their contribution to the community for their food.
I agree that between 100 and 500 people is probably the ideal size for something like this to work. If it's kept truly small and local, a desire to help the community that supports you would be good incentive to provide quality goods and services for trade.