Item bag and warehouse are for you to store items.  If your item bag is full or you have to store items which are not frequently in use, you can store them in warehouse.

 

Where is warehouse?
Warehouse Man offers warehouse function, and you can find him in some towns.

Store and fetch out items
Select Warehouse Man and click "Warehouse".  The interface for warehouse will appear.  Draw the items from your item box into the warehouse interface to store it.  Draw the item into your own item box if you want to fetch it out.

 

Deposit and fetch out money
Select the Warehouse Man and click "Warehouse" and "Deposit".  The interface for deposit money will appear.  Input the amount of the money in it and click "Ok".  Click "Withdraw" if you want to fetch out the money.  Input the amount in it and click "Ok".