Probably a silly question but hypothetically, if I put up 40 free listings at the beginning of the month and sell 10 of those items in the first week, can I replace those 10 items with another 10 listings at no charge, or must I wait until the end of the month to do so? IOW, can I keep replacing each sold item with a new free listing as it's sold or will I be charged a $1.50 insertion fee each time?