Then I don’t think you can track the amount of sales without using data stores. Because While I think .Sales really mean how many people own that asset, or item. And well you really can’t “Own” and developer product like other things.
If you really need this information I recommend you track when someone buys the developer product and add one into a datastore that tracks info about the developer product, Unless you can Figure out a easier method, like if there is some sort of error in that script I didn’t catch, or something else entirely.