Of course, as with any item, especially ones of this type there will be fakes available. (I also find that there are a lot of Sony fakes out there in comparison).
The best thing to do is use common sense, remember if the price seems to good to be true is probably is. Don't be put off by this however, go with a reputable seller (i.e. high and positive feedback [remember to check the number of retracted feedbacks as there is a sneaky way people who gather a lot of negatives get them removed]) and take a chance. After purchasing it, and before leaving feedback test the card. There are numerous guide son identifying fakes so i won't go into it but it usually in essence is checking the real capacity and checking to see the PCB board from the outside.
Do some research before buying if you don't want to be ripped off, but bear in mind it is usually possible to get a refund either from threatening the seller or filing with paypal so don't be put off entirely.
Matt