How do you sell fish in dragonfable?

If you mean the pet fish then talk to Grammy in the pet shop and comlplete the pet quests (one of thems goldfish).

If you mean fishing for fish in ur aquarium then go to falconreach fishing (near valencia) and buy angler armour. Then wait till the waters all thrashy and click anywhere on the screen to pull out your fish (or boot or stick). You can view them by clicking the side arrow and then clicking aquarium.