It's important to consider the existing system and the cost and risk of changing it. I work with a database that stores inventory kind of like yours does, but it includes audit cycles and stores adjustments just like receipts. It seems to work well, but everyone involved is well trained, and the warehouse staff aren't exactly quick to learn new procedures.
In your case, if you're looking for a little more tracking without changing the whole db structure then I'd suggest adding a tracking table (kind of like from your 'transaction' solution) and then log changes to the inventory level. It shouldn't be too hard to update most changes to the inventory level so that they also leave a transaction record. You could also add a periodic task to backup the inventory level to the transaction table every couple hours or so so that even if you miss a transaction you can discover when the change happened or roll back to a previous state.
If you want to see how a large application does it take a look at SugarCRM, they have and inventory management module though I'm not sure how it stores the data.
