This is the way the "conservative" members of the U.S. supreme court got around the "penalty" for not getting Obamacare, by calling it a "Tax". So now the antiguners are finding this may work to make it more difficult to sell guns and ammo.
It can be argued that a tax does not regulate the sale of a firearm or ammunition ( I disagree) and any city or county is able to raise taxes as they see fit.
If this action by Seattle makes it to the state supreme court and is found legal then look forward to similar taxes in every anti gun city and county in the state. They all are, if nothing else, looking to raise revenue.
This hurts the poor more than anyone. Which is the history of gun control. "We gotta keep them peasants away from getting weapons"
Buy ammo on line while you still can.