I understand both sides of this discussion. Generally, stores do not make the killing people assume they do on fish, so money shouldn't play into it. Besides, when a customer must trade in a fish they can no longer house properly I feel they lose the moral high ground. If they got into this hobby to make money they picked the wrong hobby! If an individual is discouraged that a LFS will not give them the credit/payment they think the fish is worth just remember it is a hobby and there are NO guarantees that a fish you think is worth money is worth the same to other hobbyists. Certainly there are some crummy stores with crummy employees out to rape your wallet - but this is (contrary to popular opinion) the exception, not the rule. Also important to the discussion is available tank space. Some stores may need that space for fish that will "move" faster.
On the other hand, the good will gained by taking in fish is certainly more than worth any diminished profit for the store. I encourage most customers to attempt to sell their fish in the newspaper or on-line communities. Sadly, I have seen only one customer care enough to do so. They generally just want to get rid of the fish the easiest way - the LFS.
J