Avodah Zarah 39 - Who is Trustworthy
A fish that develops fins and scales only when it matures (such as the one called "sultanit") is kosher. Nevertheless, one needs to buy it from a trustworthy seller, who can distinguish it from similar non-kosher fish and take the non-kosher ones out. If it has fins and scales but looses them when it is caught, it is also kosher.
A knife that was used for cutting a non-kosher fish should not be used for cutting kosher fish. Even though the flavor added by the knife, if it is detrimental to the taste, would not make the fish forbidden, in this case it enhances the taste.
A servant of Rabbi Levi used to re-sell food products and was considered trustworthy. After Rabbi Levi died, Rabbi Yochanan said, "A servant of a trustworthy person remains trustworthy even after the master's death - unless proven otherwise." In the same way the wife of a trustworthy person is just as trustworthy as her husband.
Art: Adriaen van Utrecht - Fishmonger's Stall