large fish are hard to sell due to people not being able to house them. my LFS has my 20"+ arowana and clown knife for sale for 6 months now, tons of customers just nobody wants to take on such a massive fish. shipping large fish is extremly expensive and risky because if the fish dies in the process, you lose out on all the time and money invested in the fish, which is always more than you can sell the fish for anyways.
good point....they do move slow sometimes......not much of a market, very few people have 300+ gal tanks....