Even when I was staff I answered this question on more than one occasion.
./sell all will never work because the prices on essentials are different than those on the signs, if they added the /sellall command then that would make the signs useless, and perhaps even the VIP area.
I don't know about staff thoughts today but I would still say no to this simply because it breaks what skyblock has been, you have to earn that money and I feel selling it through signs gives it a little more uniqueness and just feels better than watching all the items disappear in one command.