Usually the better the condition of the fish, the more the LFS will be able to charge for them, therefore the more you will get out of them. Don't "burn" your LFS by selling them sick or non-quality fish, only something you would buy yourself.
Many times, "major" pet stores will not buy fish from individuals. You may be able to talk them into "taking the fish off your hands" if it is something you just want to get rid of. Your best bet, though, is an independently owned "Mom & Pop" type fish store. Most of these are willing to give you store credit for any fish you may have....a good way to pay for your supplies!