Please know that purchasing items does not mean your attack and defense points will or should increase. Your BEST option to increase your stats is to add more mob members or use skill points on your upgrade page.
Purchasing hundreds or thousands of items does not help as you NEED mob members to carry the most powerful items in each of the 4 item sections. A combination of purchasing or earning items along with adding mob is the best way to see an increase in stats. Only doing one or the other may result in small increases that may not be visible.
Alternatively, you can invest Respect Points in to Booster Packs to increase your stats instantly.