Chelsea didn't really need to sell him, did they? And there were rumors other clubs wanted him.
What you all seem to forget is we have a transfer ban hovering over our heads, and all other clubs will use this as leverage. Cesc was, at the very least, sold for the price he was bought.
It's a terrible price and we should have much more. We don't need to sell Cesc either, it's because we want to same with Chelsea and Luiz. The ban only affects us in buying players not in selling them.