I've found that people often don't know what their plants are, so I've always brought reference books to swaps, with my name clearly labeled on the front and a note that they are not part of the swap items. It also helps ease the pain and make it less personal if realizing one or more of them are invasive weeds. When I organized swaps before, I brought those sticker name labels so we didn't have to try remembering all those new names at once, and people also wrote their forum handle on their tags. I put a label for each of my plants in a baggie kind of tucked in the pot with plant name, and my email address for questions. Also handy to take a couple jugs of water, like milk jugs, in case there are any plant-thirst emergencies. A cheap plastic tarp or even table cover makes an excellent liner for the trunk of the car. I like milk crates for moving around more than 1 plant at a time, and they help plants stay upright while driving, don't melt if they get damp.
