What if it also showed stacks in storage, regardless of market. That could also apply to commodities.
that's what i was thinking as well, showing the total amount of a good or weapon i have in storage somewhere would be more useful to me than showing the total amount i currently have in my cargo holds.
with some rare weapons especially, i like to keep a few of them around for when i need them, often even buying as soon as i find one anywhere in a market, even if i don't plan on using it in my current loadouts. but it becomes difficult at times to remember how many of those i already have, if any, and consequentially whether it's worth keeping/buying the one i'm currently looking at.
adding two lines of text to the tooltip like "you currently have X amount of this in your cargo holds" & "you currently have a total of X amount of this in storage somewhere" would solve that.