The person who hosts is the one who makes the rules but usually they are few. I am including a link to a post by Trianglejohn who host one or two swaps each year in the Raleigh area. If you read through it you may find something to help answer your question. Also you can do a search on Gardenweb for 'swap rules and come up with several links, unfortunately most of our links are dead right now while our systems are being worked on.
Hope this helps, one transplant to another (I did it from Canada 30 years ago and have never regretted it)

Here is a link that might be useful: WANTED: How do you really feel about swaps?

I host the Raleigh Swaps and I've noticed that many gardening organizations in this area now do their plant exchanges in a similar manner. Back in the old days people organized swaps with all sorts of rules (bring a plant - get a plant or first come/first served). We don't do that. We try to keep rules to a minimum and chaos to a maximum.

The one thing I have observed over the years with new swappers arriving at their first Raleigh Swap is that in the beginning it is quite intimidating. We get about 45 people, all with glazed over eyeballs and foam dripping from the corners of their mouths. These people simply love to spend the afternoon with a bunch of fellow gardeners, sharing advice, sharing ideas, sharing plants. I often have to explain that as a group the bulk of us only see each other twice a year (at the swaps). We may act like we see each other every weekend but that just isn't true. Some of us have been coming to the swaps for years and we know what others are looking for or what sort of gardening they do. More than once a special hard-to-find plant has been given to me by someone that read one of my posts. I think sometimes they actually went out and bought the plant just for me as a way of saying thank you. I cannot say why these afternoons are so much fun - they just are. When gardeners get to a certain level of expertise they enjoy propagating their plants so they are constantly looking for people to take away all those rooted cuttings. It isn't always about new or rare plants. Many times people are looking for more of something they already have, they just want more of it. Sometimes they just want something their grandmother grew. You never know. Bring what you have, what you are proud of,, someone will want it and I guarantee you'll have fun.