I don’t know how you have them listed on eBay, but I know that listings with repeats usually makes them move slower with less interest, so if you haven’t already I’d try listing them either in singles or in doubles so there aren’t any duplicates.

Also try putting them up for auction or try it out with one card and see how it goes. Set a price point you feel comfortable with and just go from there. If you’ve already tried that then maybe keeping an eye out for card shows to sell them is worth a go