There is a silent dread of obtaining a 3rd age item that sells for higher than max cash and being forced to attempt to sell that item to someone who is more than likely a scammer.
I know this would be solved by just paying attention, but the chance of losing a high value item to a scumbag could be entirely circumvented if the player could toggle platinum while buying and selling on the grand exchange.
I don't think it'd even need a toggle. Just automatically convert it one it reached a max cash stack. And automatically pull from platinum tokens in your inventory to buy stuff.
"If anything is too hard; Quit, before you find yourself a failure." -my dad, rocket scientist