I'm open to trading him but you must get equal value.
Everyone talks about the Tkachuk trade and how it changed the Panthers... well you can be Calgary in that trade too and get absolutely fleeced.
With Marner, there are a few options:
1.) Trade him for equal value
2.) Let him walk and throw that money on the stronger players in the same UFA cycle (Rantanen, Draisaitl)
3.) Re-sign him at a fair contract. Marner isn't a bad player and he is important to the team but not at the price he wants. If he's willing to sign for a decent price and there's no fair value trade or UFA - you're better off keeping him.
You don't want to be emotional and get fleeced on a trade. The Leafs trade of Phil Kessel for pennies on the dollar won Pittsburgh 2 Cups. The Leafs were rebuilding so it didn't matter. We won't be rebuilding here and if trading him makes us a worse team, then you keep him. You don't get players like him, even with all his faults, often.