Exactly... in fact hacked/false flash memory is HUGE on ebay right now. Both in card formats and usb drives. Personally unless you know a seller sells real items don't even chance it. Getting your money back is very difficult, even with PayPal.
There are a lot of sites regarding the scams, such as:
FakeFlashNews
Basically someone in China/Japan/Taiwan (Basically overseas where there is little control) gets defective memory chips and reprograms them to read as normal working chips. They go as far as to recreate the original packaging as well. The key to detecting a fake is NONE of them have a serial number!! Ask for a Serial Number before you buy.. if they can't provide one, it's fake. If they claim it's 'inside the package', odds are it's fake as most memory manufacturers now put stickers on the outside to combat this exact problem.
Use the program someone linked a few posts above to test your card.. or simply fill it up with a bunch of files to see if it's really working properly. An easy trick is to find a video file approx 1 gig in size, and copy it a few dozen times. Then load em up onto the card/chip/drive. Most of the time it'll fail along the copy process (which can take a long time for large drives!!), but if not.. it almost always will fail on the READ process when you try to view them.
Not all Ebay sales are scams, but if the price is Extremely lower than the manufacturer for these higher capacity chips.. odds are it's fake.