You can sell items directly on the Merge Board. To do so, select the item you want to sell on the board by pressing on it. You will then see a description of the item and the amount of Coins you will receive for selling it. 

 

 

When selling an item, please remember that some items, especially producer items, are much harder to get than others. After selling an item, an "undo" button will be available for a limited time. Tapping this will return the item to you and remove the Coins you gained from the sale. This option is only available for the last item you've sold.