You should be able to lock your various weapons/materials into your inventory so that you dont merch them on accident. This could speed up the merching process by a lot. You would lock the things you want to keep, then hit something like 'Sell All' then everything but the locked things could be sold the same time.
That's why they make the little toggle option. Like when your on the Sell Merchant screen it will have little thing you can check. So you can view with bag, belt pouch, backpack, and blah blah.