Well, let's not go overboard when rating Marchisio now, but I agree he's part of our team's core and I'd hate to see him leave. Also, Bonucci, who was mentioned as a 'last resort' bargaining chip, is becoming just that and shouldn't be sold. We can't afford to add quality players on one side and let go of players who have brought us such success last year on the other.
We have to invest to up the quality of the squad, not replace the good players. We should be looking to get rid of players we no longer need, players like Elia, Krasic, Quag or Matri ( for another, better, striker ), Padoin.
Let's not start shooting ourselves in the foot by selling the wrong players though, we're finally building a good squad.