Frankie's Fishing Emporium only sells raw shrimp, herring, sardines, and mackerel. You need 30% favour to buy from this shop or else the stock of all fish are 0. Higher level fish will become available at higher favour, and sharks will be available at 100% favour. Any other items sold by the shop were sold to it by other players. This makes the shop useful for getting rid of unwanted fish, as it will buy them for a higher price than other shops.
Beware, as if you are trying to sell fishes at a large quantity the price per piece will decrease tremendously.