Best way to sell an amazon card would be to first prove that it wasn't carded, and once you prove that it has been obtained legitly, next step is to agree to a price, youd be willing to sell. Good price to go with is around 85-90%. 70-80% should allow you to sell it fast enough.
I really don't think this is possible. You could always have purchased the gift card with a stolen credit card and have possession of a physical gift card. You also could have purchased one card via legit means and others via a stolen credit card or other illegal means.