Inventory List is a ready-made app for keeping track of what you have in stock.
Add each product with its SKU, how many you have, the level at which you want to reorder, your supplier and what each one costs you. Tap plus or minus as things sell or arrive. The top of the page shows how many items you track, how many are running low, and what your stock is worth; "Low stock only" shows just the items that need ordering.
A few sample products are included so you can see how it works; remove them with one button when you are ready to add your own.
It also adds a "Low stock report" task. Run it from the Tasks tab and an assistant writes a page into your Brain listing everything at or below its reorder level, with a suggested order quantity for each - it only reads your list, it never changes it.
Everything is saved on your computer.