Adult fish can either be worth lots or worth nothing, depending on your local situation. Often, it's best to concentrate on finding a good home rather than a fair price; if a local fish club has a member that will rehome an unwanted fish, that's perhaps the best thing because at least you'll know it'll have a good life. A pet shop might give you some money for the thing, but who knows where it'll end up? That's not such a big deal with guppies and other easily maintained species, but something like a fire eel is precisely the sort of fish that needs very special care.
Cheers, Neale