Create an invisible layer watermark

I list vintage large format reproduction photographs (scans from original negatives that I have spent years collecting) on ebay that I have spent a considerable amount of time "cleaning up" and enhancing. Ebay does not allow watermarks. Anyone can right click on my photos and create/print their own copy. I read somewhere, but now can't find it, instructions on how to create a "hidden" invisible watermark layer that cannot be seen except when someone right clicks on your photo to copy it. Then it becomes the layer that they are copying, not your actual photograph. Does anyone know how to do this? I am using Photoshop Elements 15. Thank you.