Sold CEX store my phone as a grade B. They now selling it as a grade A.

So few days ago sold my phone to CEX. It was quite new but however I wanted another one so got the quick cash. I didn't have the headphones as I forgot them at home. They said then that would be a grade B. So that got me £15 less than a grade A.

Today I went to check out of curiosity how much they selling it for. I got confused when I found out they are selling it as a grade A.

Are they allowed to do that? If not then I will definitely go and speak to the manager tomorrow.

Can they buy at a different grade and sell at a higher grade?