It's a common mis - beliefe that because a fish is larger, it is now more valuable. Often times that is not the case. I deal with this regularly at my shop with people wanting to trade fish in that are now to big for thier aquarium. Here's a couple things to consider...
THe reason most people sell thier large fish is because it has grown too large for thier aquarium. THis is the same reason that people who see it for sale in a pet shop won't buy it, it's too big for many peoples aquarium.
Because of this, often times a large fish stays at the aquarium shop for a long time. THey are expensive to feed and take up space where fish that are easier to sell could have been housed. These are both money loosing situations for the shop. THe shop is in business to make money, it's common that an experianced shop won't pay out big money on an item that might be a hard sell.
On a tought sell, I sometimes will do a consignment with an open end set of terms. For example, I'll take the fish for free and offer a % of what the fish sells for when it sells. Another option I have done (with a nurse shark) is we agreed on a selling price of the shark traded in, agreed on the amount (%) that the owner recieved when it sold and we agreed on the daily cost to feed the shark and daily deducted the cost of the food from the amount the owner would have recieved when the shark sold. Believe it or not, even under these terms, I lost money on the deal. I had the shark so long that the amount of money spent to feed it (not to mention house it) exceeded the the amount I would have paid the guy for the shark, he didn't want it back so I sold it real cheap ($50.00) just to get it out of my tank.
I won't take large nurse sharks anymore, not even for free.......