Hi! You can sell Silver in any City. Consider, for example, Busan. Open the City page → tab 'Store' in top. Push button 'Inventory' in bottom, this will open 'Inventory' window. Find Silver there, once left click on it and drag-and-drop Silver to tab 'Store' in the background.
Next window with icon of Silver and text 'Please enter the number to sell' where you can change quantity, apply or cancel.