Purchase Order Option should be added to track all processes of a buy/sellcycle
You have options to 'Sell' items only, however this is not the full picture of what is required to make a sale. 3 steps should be included: Estimates - PURCHASE ORDERS - Invoice.
Example: If I am buying an item from a supplier to fulfill an order for a client, I need to generate a Purchase Order after I receive the suppliers estimate. This tells them I want to buy the items on estimate. I will then sell these items to a client and generate an Invoice for payment. You should add a Purchasing Option so the entire purchasing/selling pipeline can be tracked together.