The issue with being able to spend balance whilst it's pending is that a HUGE chain of transactions via spent 'Pending Balance' will be created.
Then if 1 person in the chain trade reverses, all the trades after yours in the same chain will have to be reversed as well. It will lead to a whole lot more total reverses and probably a lot more upset buyers.
If this feature was added, would you be more annoyed that you have more reversals, or will you not care if it means you can also buy more items way faster?
As great as it would be, I don't think this would ever be added.
Said this before;
But waiting 7 FULL DAYS for balance is torture 🥲