Just need to have the patience to double check you are getting the right item. I've made the mistake(with Esoterics when they were the capped tome), but I learned from it to slow down when buying items like that. Does it suck? Sure, but it's not the end of the world. Learn from the mistake, and do better checks next time. Plus, as others have mentioned, there is a pop-up that appears if it's an item not for your currently equipped class that requires you to confirm you want it by clicking the check box, then clicking purchase.