Just put down what fish you have, their size, add pictures, and put your location in your user info.
Do not put your address or telephone number here. Tell people to private message you for information about picking them up.
Most pet shops will take unwanted fish. When the young are a few months old and look like the fish at the pet shop, ring a few shops and see if they buy fish or will take them for store credit. If you can't find a shop to buy them, see if the shops will take them free of charge.