Remember that it is against eBay user terms of service to sell counterfeit items, so that person shouldn't even be selling KOs in the first place. I would contact this person and say that it's come to your attention that the product is a fake and that you thought it was genuine. There was nothing in the auction description to let you know it was fake, and furthermore it's against ebay ToS to sell fakes, so it's perfectly reasonable for you to assume it was genuine.

Report the matter immediately to eBay and see if they can cancel your transaction or arrange to have your money refunded.

Be aware that even if the original Shockwave mould hasn't been destroyed or lost, it is a NON Takara mould which has not been acquired by Tomy and as such will probably never be reissued. Other such moulds include:
+ Jetfire
+ Deluxe Insecticons
+ Deluxe Autobots
...but a lot of Takara/TOMY-owned moulds have been KOed too, so be careful.