Ores, armor, and weapons tend to over saturate the massive market due to players considering them valuable from past minecraft expiriences. Due to numerous reasons, the massive market has a very small demand for these items, therefore, they sell for very little. Now, if you really want to make those big bucks, you need to find a niche. For example, rare building blocks will sell for a small fortune if you find the right person/group to buy your goods. You just have to keep nosing around for that item everyone wants, the market is CONSTANTLY changing.