Fish

Question

Is there anything I need to know before buying fish in a supermarket. I assume I need to ask the man to clean the knife before cutting, but anything else? I have been told that it is not proper to buy fish in a supermarket unless you can see the entire fish. But the fish in the supermarket have already had their skin removed so this is impossible (and there is no kosher fish store near me). Is there a kashrut problem here, or do we assume that when the merchant tells me that this fish is Chilean sea bass, that he can be relied upon?