Ebay is your best option. There are other sites such as virbank where you can list your cards and people will make offers on them. However, ebay will get you the most exposure and the highest prices. You will have to do a little more work, but with patience, you will earn the most money.
Yes, that is the opportunity cost. The fee’s are the cost for having the most exposure. I am sure you can list a few on ebay, maybe sell some on virbank, but you will do well with either method.
eBay will yield the most. I have moved away from FaceBook selling groups and returned to eBay recently because cards are earning by far the most money on eBay at this time.