In Minecraft: Dungeons, you will collect many items as you explore the world and fight your opponents. Most of them, however, won't be worth of your attention. It's a good idea to collect everything that drops as loot and then sell all unnecessary items. This way, you can get a lot of crystals which can be used to buy cool things later.
Selling items is very simple. All you have to do is to open the equipment & inventory screen and choose the salvage option.
The price of the item is specified in the right corner. This way you will know whether it is worthwhile to sell this particular item or maybe it would be a better idea to develop its power.