Let's say you do a schematic or some new ship design and take it to Paramount and try to sell it to them, and it says 'Star Trek' and 'Photon Torpedo Tubes here ->" all over it, it is STILL YOUR PROPERTY.
Whether or not Paramount sues you for sticking it on your website is their business. IT IS STILL YOUR PROPERTY. No one else can use it other than you. Not even Paramount.
You unfortunately won't get any response from eBay unless/until you get a lawyer to send them a letter, but once you do, they will respond.
You have the original files on your computer that are dated before anyone elses, this is proof enough that IT IS YOUR PROPERTY. You don't even need these unless/until someone challenges you in court.
Find a cheap storefront lawyer, pay him $50 to sign a letter to eBAy telling them to desist. If you don't want to pay $50 then it's not worth it to you to protect your property, once enough people steal it and you do nothing to protect it, it will end up public domain anyway then.