I had two heavily loaded WDW gift cards and had a very hard time selling them. I discounted them and tried to sell them locally so I could have an easier time proving my identity and lack of scamminess.
People were very suspicious, as they probably should have been. Several folks were contacted by my friends, who were friends of theirs, and who were going on a WDW trip.
They just couldn't let themselves buy the discounted cards.
I finally sold them to a friend who was going and she loaded them into her WDW card account. HOWEVER, when she went to use them, she needed the codes off the cards and she had lost that part. Luckily, I had photographed the cards, front and back.
Make sure you take care and possibly look into selling them on a well-known gift card site.