I have the same problem with items as Bunny - they have to be in multiples of 5 or maxed. Usually I will have only 10 on hand until endgame where I might max it out. If I ever find myself at a store with >20 or a total that isn't 5/10/15/20 then I will sell them, or buy more to reach the next level.

Lately I've developed the habit of not selling anything. There's plenty of games nowadays that have crafting systems that just screw you over for selling the most basic items. Oh no I can't get my sword of badassity because I sold that rusty fork back in the first town.