I had ordered, from Ebay, two 64gb memory cards for a couple of pounds each - this was before I knew about the problem of fake cards.
They didn't arrive. Time went by, and after several weeks, and a week after the last date they should have arrived, I emailed the seller. The seller gave me an instant, full refund.
Today, they arrived! So I tested them. They failed badly. After a couple of gigabytes, they bombed out. So even if they'd arrived on time, I would have asked for a refund.
I really don't know how to solve this problem. The scum doing this scam are obviously profiting from it, or they wouldn't be doing it. So I guess that most people don't test their cards before giving positive feedback.
I tried telling Ebay, but their response form doesn't have an option that is anywhere near "This is a fraud".
Meanwhile, all I can suggest is, don't buy SD cards or thumb drives (same scam) on Ebay, unless it's from a vendor that you have a good reason to believe is honest. Because the Ebay "reputation" isn't working in this situation.