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YOu might have been dooped, by the Ebay seller.
I know its not what you want to hear, but the Ebay Reseller is probably not an AUTHORIZED Adobe ReSeller.
And for that reason, you dont have any recourse with Adobe....you can only deal with the seller.
So, i would return it for a FUll refund, and explain to the seller, the serial number doesnt work.
Its not hard to find the serial number, and it should be easy to use and it should work.
I can however, do a remote session with you, and see if i can get it to work.
But you will have to take pictures of the serial number, so i can SEE it for myself.