Selling unwanted gift cards online is a huge scammer magnet. I tried to sell a $50 card for $40 recently and wasted a bunch of time with people sending me fake PayPal confirmation messages. Thankfully all of the scammers were pretty stupid and it was easy to tell when they were scamming.
It's just super risky on both sides so very few legitimate buyers want to do it since as a seller you could be giving them fake or used codes.