Unless (a) the sale of fireworks is prohibited, (b) fireworks are wrong in and of themselves and therefore can have no moral or lawful use, or (c) the owner is

Unless (a) the sale of fireworks is prohibited, (b) fireworks are wrong in and of themselves and therefore can have no moral or lawful use, or (c) the owner is not just selling fireworks but advocating their unlawful use, I see no issue with selling fireworks in a county where setting them off is prohibited.