It would make sense for both teams to sell those players. For Barca, this sale would help them keep Messi in the team. Barca has a lot of financial issues and they need a lot of money to offer a new contract to Messi without breaking the financial control rules imposed by LaLiga. They will either need to make very important sales, or they will need to say goodbye to Messi.
Madrid, on the other hand, is looking to purchase Mbappe, who is probably the most expensive player in the world right now. They have a solid defense with Nacho, Militao, and the recently signed Alaba, so selling Varane and then using that money to help pay for Mbappe seems like a good choice.
I feel Messi should find his way out of Camp Nou and try a new adventure too. The only issue is that there is only few teams that can afford him at the moment. It would favor Barca too in terms of their finance, they'd be able to manage their finance if they stop paying huge amounts on L. Messi.