An extortionist is a person who commits extortion: they threaten people to make them hand over money or something else. Police reports and news use it. Though actually, you will also meet extortioner, which means the same person; both are correct, and extortionist is very common now. Do not use this word for a shop that only charges high prices — that complaint belongs with extortionate. This word is for the person doing the threatening. It is a serious, crime-story word, not a joke name for a greedy seller.