Bad ebay experience Sincerity Trading
I wouldn't shy away completely from buying SD cards on eBay. There are just a few bad apples in the bunch. Basically, the feedback rating needs to be very close to 100%. Even 99.0%, like this one, is garbage. The more expensive the item, the higher the rating you should demand. Buying a knockoff remote release for $2? I'd be comfortable with 98.0%; if I get ripped off, oh well, big deal. Getting a $20 filter? Demand 99.5% or ideally higher. For a multi-thousand-dollar camera or lens, I wouldn't touch a seller under 99.8% feedback, ideally 100% feedback with a minimum of 100 sales in the past year.
That said, a minimum of 99.5-99.7% should be good enough for SD cards, especially if you stay away from China/HK.