I'm not sure if this still applies, but several years back there were alot of corrupted memory cards being sold on Ebay. I would advise you not to buy from there and try to stick with a name brand.
Here are two links on the Ebay Fraud issue....
http://martybugs.net/articles/fakesandisk.cgi
http://fightflashfraud.wordpress.com/
* The second link has a focus on flash drives.