I know with SD, MicroSD, and USB flash storage (not exclusively) are commonly sold on ebay as higher capacities than they really are. Theres lots of guides on google for checking your cards to make sure they are legit.
Not that i doubt the reputation or trustworthyness of the source, its just always a good idea to check.
Also, make sure you dont lose it. Im still annoyed about my 16gig class 10 card i lost the other day, popped out on me and i didnt notice. i was not a happy camper.