I have had really good luck with ebay. I've never tried to buy something that needed to be authentic (like a purse or jewels) but I tried to sell a purse once and no one would buy it unless I could supply the little plastic credit card that came with it for authenticity.
I know Gucci watches and purses come with the credit card- what else does? If Tiffany jewelry does- you could ask for that also.
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