Just to be clear...
Is the probem with grading, or that you didn't recieve the item? If this is a problem with grading, but the seller specified that his NM/M may not be the same as other people's grading policies, I, for one, think you'll have a very difficult time proving that he ripped you off.
Also — have you attempted to contact the seller by email, or even by phone? There's not much point pleading your case on this board, if you haven't even talked to the seller (which wouldn't surprise me given our past relations). Ebay will provide you with his personal information in the event that a problem has arisen between the both of you.
Moreover, while you may not want to hear what I have to say, I would strongly recommend that you amend the comment regarding "sending only paypal or credit card" to Argentinians. Some liberal-minded people might take offence to that remark seeing as it implies that all Argentinians are crooks like this seller.