Some pet shops buy fish from customers, otherwise you can sell them online at Ebay, Gumtree, etc and put down "pick up only".
See if there's an aquarium society in your area and take some to the meetings and donate them to the raffle table. You can also tell people at the fish club you have more for sale if anyone wants some.
See if the chain stores will buy them from you. Even if they only give you a credit for some fish, plants, food or ornaments, it's better than nothing.