[jaredsohn]

I was going to write something similar to this. One issue, though, is that it doesn't necessarily track new purchases.

To do that as well, it needs to be an extension and it should also monitor whenever you buy something. If there is a concern that purchases might happen when on another computer, you could allow the user to enter their password into the extension so that the extension can monitor things for you in the background. While users don't have a guarantee that the extension is using the password securely, at least it is possible for the source code to be inspected.