That's weird. I thought this price is cheap enough.
I would have considered buying it, but I just picked up a $50 card last week, which will last me for a while, so I'm not eager to buy. For some reason, iTunes doesn't seem to sell all that well on here. I've bought several, but usually at deep discount from folks that can't sell them to anyone else. Because I'm a bastard like that.
Good luck with the selling, and I hope you find a buyer.
(Oh... one other thought... people might be hesitant right now because of the big price swings going on in BTC. No one wants to buy a $20 card and then realize a week later that the BTC they spent are now worth $30.)