What does a quest item's tooltip say when you are in a merchant menu? "Item cannot be traded here." This suggests to me that maybe the item should be tradable somewhere else.
What if they made quest items farmable? When you use a quest item, it doesn't just go away, it gets banked as some point value, and then there is a shop where you can spend those points. Now, I don't necessarily mean that you just rerun quests. Maybe you have to start a new character and play through the quests, but maybe not. I think an argument could be made for either. Maybe normal quest items are worth 1 point each, 5 for nm, maybe 15 for hell; of course the values would be calibrated in some way.
What would those points be spent on? idk, maybe more stash tabs? Shiny item skins? Rejuv potions?