My suggestion to you would be to create a requirements document.

Some simple questions which need to be answered before starting the database design would be:

1. What model of a system - first in, first out or last in first out or both flows be implemented with first in, first out for products which might have a "use before date". Or the third option which is neither first in / first out or last in/first out.

2. Is product cost to be tracked in the inventory system, so that the inventory value can be arrived at for fiscal period accounting purposes?

3. Do you have to track which item and number of, are shipped to what outlet (store or customer)?

Additional items added at 12:35

4. Will the inventory system have to calculate and set re-order levels for stocked items? Then when stock on hand is at or below the re-order point generate a report for purchasing.

5, Are stocked items seasonsal items / non-seasonal or a mixture of both. Seasonal would be something like bathing suits, while non-seasonal might be nuts and bolts.

And as you familiarize yourself with the complexity of a good inventory system you will find more questions that must be answered before starting the design of your database.