edit: you can sell in batches of 100 only but very fast if you leave the key selected or mouse left click selected over desired resource that want to get transactioned.
however, it’s very fast not accurate batches of 500 so be careful it’s crazy fast but it works!!
At first I wondered this too. But then I find out, you don’t need to click, there are hot keys, for example Z to sell 100 food, and it continuously sell when you hold the key. You can just press down Z for like 10 seconds to sell 10000 food. I find that also acceptable.