The items on the mainboard can be easily sold within the game. In order to do so, please tap on the specific item and click on the 'Sell' option that appears above.

Note that the sold item can be brought back by tapping on the 'cancel' option that appears right after selling. However, this option does not last long and once the 'cancel' option disappears, the sold item cannot be retrieved. Thus, it is highly suggested to carefully choose the item to be sold and make sure a high-level item is not selected.