When buying a memory card on eBay, you need to do your homework or possibly even reconsider buying one off ebay as around 95% of these items appear to be fakes.

First you need to thoroughly review the sellers feedback and avoid sellers from China/Hong Kong. A lot of these cards are listed as 4GB but when you use it it may only hold 2GB. They sometimes don't even use real memory chips. It could be a Sony Card using a Samsung chip.

In conclusion, in order to buy memory cards safely, see if the seller has good feedback, check for a refund policy, pay with PayPal and test the card immediately by transferring a large amount of data to the card to see if it reaches its full compacity and if possible test the card with a test available online such as the "magicgate" for sony magicgate cards.