it would damage the global market and the average market index would fall or rise by a good $10,000. This is because it would cause less people buying from global and more transactions to alliance. Then prices between alliance trade and global trade would compete. They would all fall. Small and Medium nations would find it difficult to sell.
Also Larger alliances would have an advantage over smaller alliances as they have more resources. Also resource tax rates would increase to allow greater trade. PnW would turn from a politics game into a business game.
Not a bad idea though