If you’ve not seen Storage Hunters on Dave (or the equivalent channel in your country) then you are missing out. This activity uses the concept of someone buying something and selling it on, hopefully for a profit on their part.
The students must read carefully then calculate the percentage profit or loss.
KS3 Maths Curriculum Area
Ratio, Proportion and Rates of Change Solve problems involving percentage change, including: percentage increase, decrease and original value problems and simple interest in financial mathematics
Andy Lutwyche is a purveyor of mathematics and a producer of resources, more of which you can find on his TES shop. You can also follow him on Twitter at @andylutwyche.