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.

but he sold them to people outside the country, where using fireworks is legal, right? it was those people that sold them back to where it is illegal... (if i understand correctly).

It would and should be perfectly ok to manufacture fireworks and run a wholesale fireworks export operation in such counties.