barcode inventory design - transaction or master?
Your sum idea works to a point, and then you will end up with a slow transaction whenever you add or remove items from stock due to scalability issues.

The way I did it for a similar problem, was to maintain a running total table, which would work in your case.

I had an audit table as well for accounting purposes, which had original values in case the accountants got twitchy, but the running total option is more scalable.