This is a minor thing but while mining it can alleviate some annoyances:
When you use /shop to sell junk blocks, the first item slot used is the quick bar slot.
Like most other people I guess, I find useful to keep a few blocks to make bridges or to repair tools so having to replace said blocks in the quick bar each time you sell, going under 64 items, is annoying.
Explanation:
If you had 74 cobble blocks, 64 in the main inventory and 10 on a quick bar slot, when you sell them the count starts from the quick bar slot and leaving it empty. Since you needed those 10 blocks in the quick slot, you'll have to reassign them from the main inventory.
I don't know if this has something to do with the game code at all, but would it be possible to leave the quick bar slot for last? This way if we have 10 blocks in that slot, they will stay there.