I think it's probably the demand. I've found that people are only willing to pay about 85% of face value for gift cards and I've sold over $1,750 worth of amazon cards on this site. The reasoning is that there isn't really a benefit to anonymously buying gift cards. There is however a good reason to anonymously sell gift cards (stolen, to make an example) and that is why customers demand such large discounts on these cards. To give an opposite example, that is why I believe precious metals for bitcoins is a good business. There is a good reason to anonymously buy precious metals (scared the government will confiscate them) and selling them for bitcoins isn't illegal (advantage over selling illegal products/drugs for bitcoins).
Yes, I have noticed that about this forum. I myself would be extremely cautious to purchase gift cards from anyone else because of fraud and the like. I'm selling my cards not for the big discount seekers, but for people that want to be sure that they're getting cards from a reputable source (as I have never resold cards that I didn't purchase myself). Most cards I sell are bought on the spot anyway, so there'd be no way I could offer a massive discount. The discount I do offer is just an incentive for people to choose my cards over their preferred exchange method if they goal was to cash out their coins to buy the item. Of course, if I am to expect people to trust me on that same level, I'd likely need to build a lot more feedback around here, and even then, who knows.