My friend Mark has already written a useful guide on fake flash memory at reviews.ebay.co.uk/Beware-of-FAKE-1GB-2GB-4GB-8GB-USB-Flash-Drives-on-eBay_W0QQugidZ10000000000953346.
I would like to add how to check the memory you get to make sure it's okay.
1. If you cannot use it at all:
If you can't read the memory in a device and it's bigger than a unit you already have and use, it might be that the memory is too big for the device you are trying it with. Try it in a USB memory card reader before you make a complaint.
2. How to check the capacity:
Fake cards can use various techniques to look genuine. To be sure, you need to follow some instructions:
a. Get some big files that are hard to compress (I personally use .mp3 files, but other sound and video files work well).
b. Copy these files to a subdirectory on the memory card using your computer and a flash card reader.
c. If you copy to the root of the drive, you might be limited to around 120 file entries, making it look like the card is smaller than it is, so make sure you copy to a subdirectory.
d. Select all the files on the flash card and see how much space Explorer thinks they take up. My results were:
Size Stored Utilisation
128MB 120MB 93.75%
512MB 483MB 94.33%
1024MB 954MB 93.16%
e. Copy all the files back to your PC and check that the total size is unchanged.
f. Check the files are okay, eg by playing them back or comparing them to the originals.
If you get over 90% utilisation and the files are okay, then your card works properly and is less likely to be a fake.