done you like a kipper

When you make someone your fool

Legend: "Want to buy a snowboard?"
Kipper: "sure, how much mate?"
Legend: "£300?"
Kipper: "Awesome"
Legend: "ahh only cost me £200, done you like a kipper!"
Kipper: "Where does that saying come from anyway?"
Legend: "From me when I sold you a snowboard!"