The majority of cheap sd cards on ebay are fake, and will result in countless millions of people losing their irreplaceable holiday photos / school work / college dissertations etc. just so that someone can make a quick buck. Not to mention the number of shoddy "made for ebay" products which fail the second time you use them.
Ebay's feedback system is the cause, as most people leave "Excellent A+++, recommended seller. Will buy again" the moment an item arrives or appears to work when they first try it. Instead they should adopt a feedback system when you can mark an item as received, but can flag an item as poor quality any time later. This would allow dodgy products and/or disreputable sellers to be identified and avoided.
Savvy buyers ought to at least have a way to warn other buyers of poor quality, even if the majority / consensus wish to continue buying cheap fake / inferior goods.