Your best protection is to mark it in some way that a buyer would not notice. Take a photo of the item with marking and keep photos of you packaging and mailing it. You can get very cheaply pens that will leave an invisible to the eye mark that can only be seen under a certain light.

If it has a serial number post the last 4 digits in the ad and on a photo in your listing. Don't show full serial number. I have some friends who sell very high end electronics and this what they all do to protect themselves. Good luck.