Hi there,
Yes, this swap is annual event and some people do know each other. We started years ago from our houses as some people were in Tomato group, some from Wintersowing group on gardenweb. Hard to think that Esther, neighbor whose yard we used to run swap is now gone 4 years. We have been at Evanston UCE since then. No specific rules, bring what you can to share and take something you like to grow. Plants, seedlings, divisions, garden items. We like newbies and there are lots of very experienced gardeners. If you have no plants to share maybe plate of cookies for our small refreshment table will do.
Mike, I love abundance of your garden! If you decide to take a day off, visit Botanic Garden and stop by the garden swap that would be fantastic! Otherwise I will be in Palos Heights May 21 and will be happy to accept any donations should you decide to share some of your bounties. I have decent number of heirloom tomato seedlings if you grow any.