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).