Actually it would be copyright, this is your design. A trademark would be your business name or slogan. All you have to do is prove that you designed the item first; it doesn't have to necessarily have to be registered with the copyright office, but can add to the legitimacy of your claim. I would have eBay tell you exactly what the violation was that removed you from eBay, unless you still have it in writing.
Of course, if you have good documenation of the removal or any correspondence, etc. then I would forward it all to the BBB, the Attorney General and whoever else would listen. Screw eBay! And yes, they would be liable for lost wages, so I would do a little research on what these others have sold, BUT remember, as the original designer, you should always expect that your sales would have been higher, because the ORIGINALS, NOT KNOCKOFFS, should always command a premium price, at least until the market is saturated.