I wish you luck, but I think you'll be running into a wall with eBay. A few years ago, I spearheaded a similar action against some guy on eBay who was selling Battlestar-related schematics off my website on T-shirts, including a couple of my own personal designs. "RedSoxFan" was his alias, or something like that. Anyway, even after getting signatures from the other artists he ripped off, because none of us owned the rights to Battlestar Galactica, it was ultimately incumbent upon Universal to make the claim, as they own the intellectual property rights to BSG, not us, who merely made non-official designs based on their universe. No recourse.
Now, since you did the Titan, you might have a stronger legal basis than we did, as Paramount has recognized your work as a part of their I.P., albeit not quite "official" (on-screen), you have official contracts with their agents, with the books & all, and they have more of a vested interest in protecting your work than Universal did with us. So you might have a shot. I'm still thinking that eBay won't listen to you unless you whip out a Paramount lawyer on them. Good luck - I am particularly interested in how this pans out - please keep us informed.