Fish, like anything else, are only worth what someone will pay for him. How big is it, if it's too large for a 125? Must be huge.
I'd start with a pet store (not petsmart or petco, though) that specializes, or sells only fish and fish related items. If possible, take a picture of it to them, tell them how large it is and ask them if they'd buy it and for how much. This is definitely no indication, though of how much it is worth.
A friend of mine sold her fish back to a fish shop when it outgrew it's tank. They paid her 30 and sold it for 200. You can ask them how much it is worth if you sold it privately. (I'd go to at least 2 shops, they're all different). You could get an idea from them. It depends on how interested they are in that type and size of fish. If they can accomodate it, they might just want it to keep for display, and maybe you'll get more for it that way.
Also, if you check enough shops, you just might come across one who has a customer that is already looking for such a fish, which might get you a little more.
I don't think this is a common occurence, so it's not like you can check the ads and see what others are selling for, but you have to start somewhere.