I was sold a couple of counterfeit cards that had all the appearance of being genuine. Until they failed.
I'm not saying all cards on Ebay are fake, just that the chances are that they may be. I'm not willing to take the chance of losing images just because I wanted to save a couple of pounds. Work out the amount you'll save and then divide it by the number of images you're likely to put on the card throughout its life. Fractions of a penny?
I buy my cards from 7DayShop or MyMemory now and haven't had any problems.