I personally do not see any deception on the part of the seller. If the item is indeed very clean in every other respect, a 1/4" scratch on the outside of the back of the lens is not sufficient to indicate mis-representation on the part of the seller, IMHO. And you should have resolved any questions about the condition of the lens before bidding.
What happens now, however, depends on both your attitude and the attitude of the seller.
Were I the seller and you wanted a refund for the lens, I would give you the refund because you might very well otherwise give me negative feedback. But the downside of that is that I would likely return the favor by giving you negative feedback in return. And of course that would hurt both of us, and is really not worth the hassle, especially if you want to continue buying and selling on ebay.
As others have suggested, the best course of action is to attempt to resolve the issue with the seller before taking any action that will damage your reputation as well as that of the seller. The issue most likley boils down to a simple difference of opinion as to the meaning of "very clean."
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