Interesting thread!
How stuff sells at any given event depends a lot on the event itself. I don't even take these to sci-fi conventions. A those, it's my Doctor Who soaps that sell well. However, my all time best seller has to be the Cinnamon Honey Goat Milk soaps.
https://www.etsy.com/listing/65950292/goat-milk-soap-cinnamon-honey-large-bar?ref=shop_home_activeThe draw of these, other than the fact they smell great, is that in addition to goat milk, they also have real honey and ground cinnamon in them. Honey, like goat milk, is humectant as well as antibacterial, so they're supper moisturizing. Then they have the cinnamon which exfoliates as well as increases circulation. I routinely sell out of these at the generic craft shows. They move well at the local farm and pet supply store that carries my soap, and there are a few folks who buy big batches of the guest sets every few months.