How about if you could just click on the items in your mechbay and set the required number to auto sell down to after every mission.
Then you could set JJs and Medium Lasers to 20 and forget about it!
Or even just a single adjustable global maximum and auto sell for all items so you could for example never accumulate more than 50 of any item.
I like the idea, but with reputation influencing prices, too much potential for people to end up 'auto selling' at a less than ideal price due to the fact that you hit your threshold on a planet you're not very welcome at.
I DO like the idea tho, just don't see it being practically employable for every gaming instance for every gamer. The other suggestions (+10 buttons, editable integer field etc) work for everyone, without being a negative impact (potentially)
Why? because you KNOW someone somewhere will get to that situation where just 33k more c-bills was the difference between getting that gauss rifle from the black market or not but since they at system X the prices were Y so they couldn't make enough $ to get the shiny.