I was wondering if there were any advantages to building an invoicing system using separate tables for Purchases Orders and Invoices vs just one table for all Transactions...

My thought process was to create only one table called Transactions with a TransactionType field to differentiate between an Income Transaction and an Expense Transaction. This way, the SUM of all or a found set of Transactions is essentially the Balance, showing Profit or Loss for the found set (or all). Of course doing this creates the need to show Expense Transactions as negative totals and Income Transactions as positive totals (much like a bank account ledger).

This also allows me to add to and subtract from my InventoryItems::QuantityOnHand, thus efficiently maintaining and tracking Inventory Items.

Does this make sense to anyone else and/or is my logic flawed?

Finally found the post I thought of when i read your question. Take a look at Rod's ERD.