These days there's no way you CAN be careful enough with ebay with such things. Stick to Amazon because of their return policy.
On eBay black market items are everywhere and they DO look precisely the same. As a previous poster noted, an SD card can claim XMBps but be substantially lower.
The reviews ---will---not---help---you--- because the consumers are so often fooled that they are willing to give positive reviews.
I discovered this when I bought a large number of Energizer batteries from someone with absolutely stellar reviews. I got them and they looked precisely the same (down to the pixel!!!!) but tested roughly 30% lower on a battery tester and roughly 40% less longevity (a rough feel) in real life usage. So I did some research and discovered that the battery companies are well aware of this can do very little. In fact, black market batteries are a thriving business. And it doesn't end there.
This is true of most rebrandable small items such as batteries, memory, watches (been a thriving industry for decades), etc., etc.