If the promos stack, keep them all in [slot1]. The quantity on the sign should be set to '1'. To sell multiple promos that do not stack, fill the entire sell chest with dirt except for slot 1. Use another chest/hopper connected to your sell chest, so that the items will feed into your sell chest. This configuration also is the way to sell enchanted books, enchanted tools/armor, and horse eggs! Good luck with the sales!