One of the biggest mistakes newbie soap makers make is trying keep every potential customer happy. Every weekend there will be a potential customer that asks for a soap that you do not make. You will be tempted to make this soap during the week. Then you never see that prospect again and your stuck with a soap no one else wants.
Keep a pad with you during sales weekends and track all requests. Once an item appears three times on your list, then, and only then do some extra surveying of customers to gauge demand for that item. Try and get some pre-orders before you make it. The promote it on your table or in your store. With-in a couple of weeks you should have your answer. Remember, be slow to add items, but once added promote them heavily to determine future demand. Good Luck!!