I disagree, I've sold gift cards many times.
as with any item sold on ebay, yes there is a chance someone can pull a scam on you, but they can do that with -any- item you list.
So far I've not had a scam attempted on my gift cards to date (Only sold 10-20 or so to be fair)
just make sure to state exactly how much is on it, when you ship make sure you have tracking, and list it for a little less than what it's worth (Typically I do 18-19 bucks for a $20 giftcard), the store its for will determin how close to full value you can get on it. For example a walmart gift card will go for almost full value, but a small non-nation wide store may go for half price, just do your research first.
also make sure you do not reveal the pin in your pictures
The cost to sell it on ebay has proven to be cheaper than other options I've seen (Granted I haven't looked all that hard)