Automation of inventory purchases start with a trigger that notifies you when new purchases are required. This trigger normally comes in the form of a data that says how many of an item you should have before you reorder the inventory. This is know as a reorder point.
Reorder points are established based upon average inventory depletions and purchasing frequencies. In other words, how many items you go through in a week and how often do you order the product (daily, weekly, biweekly, etc). With the data trigger setup you can now automate your inventory purchases if you have the right software.
Creating the purchase order automatically based upon on-hand inventory levels as compared to the reorder level can be performed in many mid-level inventory control programs like Access Your Biz accounting or Microsoft Dynamics. However there some preparation involved in order to streamline this process.
Your inventory needs to have a preferred vendor assigned to it and, possibly, its vendor part number along with last cost. With this setup, purchase orders can easily be created with a click of a button. Normally the purchasing agent will also have a preview screen before the order is finalized so the agent can easily review the order and make adjustment for quantity and vendor selection.