I've run a big fish rescue for the last 4 years. Of course, big fish have a value, which is, however, modest or even tiny relative to the effort and funds that went into raising them from babies. But, the value can be nothing too. One extreme example you provided wherein someone paid a LOT of money for one. I have never heard about another one like that. In my practice, I've not been so lucky to have people just give me their big fish for free left and right. Out of about a 100 fish I rescued, perhaps I've received only 5 for "free" (that is not counting my driving sometimes up to 5 hours one way to pick up a fish). For the other 95 I had to pay, from like $20 to usually no more than $100.
Now, the other extreme is found regularly on site like Craigslist, where people advertize fish for free. Many here on MFK say it happens a lot. I believe them. It's just I have never been so lucky. All my "free" donations came by the word of mouth.
So, to sum it up: any value?
******If you, the seller, are unlucky or have no time/desire to be looking for more than nothing in return, you get zero back.
******If you are lucky and/or put in a good advertizing effort, especially on the right sites like this one, you can count on a modest return of up to $100 (in the US) for common fish (rare fish can be much, much more expensive; O. niger is rather common).
******Finally, you'd have to be amazingly lucky to score big for your fish, insomuch that no one should take into account that possibility when planning to unload their fish. As in my analogy, when someone buys a lottery ticket with 1 in 10 million chances of winning 10 million $, they don't quit their job and start planning their retirement
