From a business perspective
If you are in the position of a retailer, making something available to the public that is desirable
and hard to come by
then you have control of the price tag
If you are in a pinch with a fish that you no longer want and are eager to rehome
The potential buyer has control of the price tag
So if you find someone who is looking for just that fish
you can probably get that $40~50 for it
But if you are looking for someone to take care of the animal you took responsibility for and can no longer maintain
Dont expect the person who does you that favor and takes up the responsibility you cannot deal with to give you $50 to do you that favor
Which is why I said if someone in my local fish club needed to rehome that fish
and I had space for it
Id offer them what they paid for it (which would be $5~10).