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.
How do you plan to get cards at a lower value? Do you have a permanent supplier selling it you for a low price?