Important note regarding MMB clicks:
There was a change made in 1.18 client, which made it stop sending the middle mouse button clicks to the server (when being in survival mode). Thus, middle clicks are currently supported only when in creative mode (we don't know why such change was made by Mojang and can't assure it will keep working).

As a result, we recommend to update your config.yml adding shift+click as an alternative way to use sell all option within shop GUIs:
You can obviously use other action of your preference (such as Q button + click known as drop) as described in the code snippet above.

In this update we introduced this change to default config.yml, however you have to update the one on your own server manually.