The classic purchase order or Invoicing system (Buy or selling works basically the same) is to structure your tables like this:
Vendors::VendorID = PurchaseOrders::VendorID
PurchaseOrders::PurchaseOrderID = LineItems::PurchaseOrderID
ProductsOrServices::ProductID = LineItems::ProductID
On your Purchase Orders layout, you use a portal to LineItems to list each type of item purchased on that Purchase Order.
The reporting that you want would be from summary reports based on the LineItems table as you can find, sort and summarize your items purchased in a number of different ways from this table.
The PurchaseOrders starter solution that comes with FileMaker is structured with this basic set of relationships so you can open the template and examine its structure.