When you have a $15 Xbox Live 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. There are various platforms where individuals can list items for sale. You can create a listing for your gift card, clearly stating the value and what it can be used for. Make sure to provide accurate details about the card, such as the code and any expiration information. However, be cautious when using online marketplaces as there may be risks involved, like dealing with untrustworthy buyers.
Another avenue is to check with local gaming stores or pawn shops. Some of them might be interested in purchasing gift cards, especially if they have customers who are looking for such items. You can visit these places in person, bring your gift card, and inquire if they are willing to buy it. They may offer you a certain amount in exchange, which could be less than the full value depending on their policies.
You could also consider reaching out to gaming communities or forums. There are often members within these groups who might be interested in buying a $15 Xbox Live gift card. You can post about your offer in relevant sections, but again, be careful and verify the identity and credibility of those who respond to your post to ensure a safe transaction.