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Josh K.buy from a reputable seller with excellent feedback and a money back guarantee. As zach said, dont buy "as is" raw cards. Instead look for a seller who guarantees authenticity. If you have questions about a seller, posting here is a good idea - you will usually find that someone on this board has dealt with the person in the past.
Finally, I also agree with zach that the more you buy, the more you will be able to recognize the differences b/t the real ones and the fakes.