To me he's one of our few genuine stars but I can see how you could justify moving him on for the mother of all deals. He's a star but he'd hopefully get you 2 good first round picks from a side like GWS or Geelong where you try to turn him back into a star mid or two.
Marshall is a brilliant player but with an average midfield along side him he's having to do a bulk of the work for them. You could make a case for selling him and using the capital and finding a more rudimentary tap ruck cheap.
That said his last month has shown exactly why you don't sell off your better players. He's the type that wins you games when in that form.
I don't think that's a case for the modern game.
Marshall is not the best ruckman out there, but he's a lot more useful, than someone who spends most of the game jogging from stoppage to stoppage.
You are downgrading your team if you go for a rudimentary ruckman.