Card displays are tricky....I bought a postcard spinning floor rack online for $29. It has like 64 slots on it for postcards (or cards). Since I dont make big cards, this works out well. I never make 5x7 cards.
BUT...its kind of a pain to haul around and takes up space. So many times I just put my cards in a box and have separators for each section with the kind of card in that section. I also picked up a shallow tray at Hobby Lobby one time for like $2 and I use that sometimes too...I never worry about my cards at these things because I rarely sell any cards anyway. The $ Tree has some bins that I was thinking would be good for cards. they are kind of open on the front, but have sides and back, so people could just kind of thumb thru the cards.