Where do you get these anyway?
My codes come from the Bing search rewards program. It takes a long time to get stuff, but it is basically free money and I've found Bing to be not bad at all.
Use the forum search to find grantbdev on the trusted traders thread. I have sold two amazon codes and a bunch of TF2 items on this forum before, but that's about it. I'm not dropping to 1 BTC at this exchange rate yet though.