Hi,

The below page may help you, however for specific services I'm personally not sure.

http://www.reddit.com/r/beermoney/comments/1a1d5e/whats_the_best_way_to_exchange_amazon_gift_cards/

If you do however decide to sell gift cards online using such services as eBay etc I would advise you to be very careful as there may be a high chance of you getting ripped off. And in most cases eBay or Paypal may find it very hard to help you on cases where digital goods are sold.

Edit:

Have a look at these websites:

http://www.cardavenue.com/
http://www.swapagift.com/
https://www.plasticjungle.com/main


Thanks,

Jack