I guess this all depends on you geographic location/climate.
If you live in an area with long, cold winters *as I do* , you've not
picked the best time of year to sell them off if you are looking for
top dollar. Most LFS/LPSs, including the one I work at, are
hard-pressed to take large fish like this even for free. If you are
able to, wait until spring-time, when demand is up and so is the
asking price. Most fish/pond stores are just looking to unload their
stock at this point and not overly-interested in taking on new stock.
(eats up profits to maintain them over the winter).

You may even have better luck dropping a few bucks to advertise it in
a local paper or put up a few ad-posters at supermarkets. Doing it
this way, you stand to probably make more than twice what a
fish-related business would offer you.

In conclusing, assuming it not to be some sort of really rare color
variety, I've seen fish this size retail for $60 - $110 in my area.

Good luck to you.