Shark Fin Vendor is a character who owns a Shark Fin booth on Main Street. If you talk to him, he'll give you a free Shark Fin. You do not need to talk to the Shark Fin Vendor when completing Shark Tooth Island.
Incredibly generous, the vendor freely gives you a costume piece, and keeps a cheery personality. With relaxed eyes and a yellow complexion, he sports a shark hat and shirt. He also wears the shark fin costume piece, and a blue pair of pants.
Upon arriving on the island, you are greeted by the vendor, who, when prompted, gives you a free shark fin costume piece. He insists that now your outfit is complete. This costume piece is irrelevant to the quest.