I dont think Derek came off as well as people imagine - there were lots of shots of him looking sly between the arguments, whispering into people's ears and slithering like a crocodile in the water. Subtle, but it will stay in people's minds.
If they all have ingredients there they are all free to make them if they want to.If they chose to trade something for ready made biscuits rather than making them themselves then it's because they think it's worth it rather than spending the time to make them themselves.