Purchases
The Purchases module tracks what you buy: orders, their line items, and what everything cost.
Orders, vendors, and line items
Each order is a purchase from a vendor, with a line item for each thing on it. A line item can link to an inventory part, and to whatever it was for, a project or a machine, so cost rolls up where it belongs. That's how you answer "what has this build actually cost me" without keeping a separate spreadsheet.
Receiving into stock
Mark an order received and the parts it brought are added to your inventory, so buying and stock stay in step.
Stock that reorders itself
With reorder points set, running low drafts the purchase order for you, grouped by vendor, with the low items as lines. You review and send it; nothing orders itself without you. Receiving that order tops the shelf back up, closing the loop. Vendors are records of their own, so a vendor page shows its history.