I don’t think there is any service that is going to trade gift cards without some sort of fee.
Cards are sold at anywhere from a 3% discount to a 20% discount (or greater)
Some are much more popular/in demand than others.
You can stand outside a retail location and offer to sell a card at a discount for cash.
You can also sell them on eBay at a discount and pay the fees to get cash and then possibly buy
other cards that you want at a discount.
**The premise of gift cards is idiotic unless you can buy them at a discount.