This is quite an adventurous step. I once worked on a project of maintaining and accurate digital record of inventory levels for a business. The challenges were astounding. So I know it is not an easy proposition to keep accurate records of inventory levels.
I recently wanted to buy 7 copies of a book from Amazon. Each one was going to a different address so this involved seven separate purchases. Amazon's automated inventory (one of the best there is, especially at their volume) began by telling me there were only 5 copies in stock.
Each purchase decremented the inventory by 1 until I made the fifth order. Then, somehow or other, there were an additional 2 shown as in stock. I'm glad I didn't believe those early reports!
So in the beginning, at least, I do expect this Google inventory check to be a bit buggy. However, with Caffeine in place, they probably will be able to stay a lot closer to present time than if they had tried this last winter on Big Daddy.