Yes, you can sell your extra items for Gold, the option to sell is located on the left-hand side of your items. Also if you acquire too many items you will be prompted to go to your Inventory to manage them.
Earn a duplicate talisman you don’t need? Press and hold on talismans rewards you earn from battles to check the stats of the earned talisman and immediately sell it if you don’t want it.
TIP: You can purchase extra slots in your Inventory - just click the '+' sign at the far right of your items.