Amazon’s ad business is blowing up. You probably knew that already. But that growth for Amazon isn’t a win for many sellers, particularly the early power users on the platform. There is heavy demand for Amazon inventory, with the average cost per click jumping from 86 cents a year ago to $1.27 today, the Amazon agency Canopy tells CNBC. Amazon is opening up huge new swathes of inventory as well. In the past three years, sponsored product ads have gone from 20% of Amazon’s top search result space to more than 40%. But the rates are still going up. And the added inventory is a boon for bigger, legacy store brands, which can box out independent sellers on the platform.