If I wanted to find out how many trash/shells/mushrooms etc i have I would have to add them up in my head (takes long time) or use the calculator on my phone(takes shorter amount of time) so I think there should be either an in game calculator or it tells you how many you have total.
Ex: you have 34 purple mushrooms and 69 mushrooms total.
Would just be an easier way of counting your sellables.