Not everything is what it seems
Speaking from experience, be sure you trust the seller before buying. I bought a 4gb stick off Ebay recently and it couldn't hold more than 1gb (even though it reported as 3.7 gb in explorer). There are many sellers out there manufacturering cheap junk in seemingly authentic casing and making a fortune.
When you get your card, I suggest copying enough files to fill its capacity. Remove it, reinsert it, then try copying the files back to your computer.
Make sure you're also aware of the steps to take if what you've received is fraudulent.
Keep in mind that more than 98% of all 1GB, 2GB and 4GB MS listed on eBay are counterfeit items and many have fake capacities.
Just want to be sure everyone's informed before committing to buy.