When you have a $100 Amazon gift card that you no longer need or want, there are several options available for selling it. One option is to look for online marketplaces that deal with gift card exchanges. These platforms allow you to list your gift card for sale and connect with potential buyers. However, it's important to be cautious when using such marketplaces as there may be risks involved, such as fraud or getting a lower price than the card's value.

Another possibility is to check with local pawn shops or second-hand stores. Some of them might be interested in purchasing Amazon gift cards, especially if they have a customer base that shops on Amazon. But again, they may offer a discounted rate compared to the full value of the card.
You could also consider reaching out to friends, family, or colleagues who might be interested in buying the gift card. This can be a more personal and straightforward way of selling it, and you may be able to negotiate a fair price without having to deal with the uncertainties of online marketplaces or other third parties.