Some sellers put a code in their titles or descriptions. There is a field on the listings page to add a custom label that would be visible only to you. I am assuming that is what it's for - I don't use it myself.
Go to your active listings page: https://www.ebay.com/sh/lst/active
On the right, click customize table:

Check off custom label and click save:

That should give you an area to add an inventory control number or note of some sort for each of your listings.
The easier you are to offend the easier you are to control.
We seem to be getting closer and closer to a situation where nobody is responsible for what they did but we are all responsible for what somebody else did. - Thomas Sowell