Simple as that, thanks.

Sorry, but I do not know which plugin add that command.

Hi Georg, all auctions plugins. Practically all survival server uses an auction house. It's a gui where people sell items.

these are the most common: https://www.spigotmc.org/resources/auctionhouse.61836/
https://www.spigotmc.org/resources/crazy-auctions.25219/

Both share the /ah sell command

My bad, this can be fixed using the in-built blacklist sytem, thanks.

my bad again, can't be fixed, anyways you can close this if you don't want to attend it, I'm not sure if I should reporting this here or in the auctions plugins.

I tested it with auctionhouse and with the default Minepacks config this worked for me to blacklist the backpack item:

backpack:
  name: '\u00A7eBackpack'
  item: 'minecraft:player_head'

for crazy auction the filter seems to be a bit more limited and requires to ban the player_head item entirely from auctions.

yes sadly I am using crazy auctions, well thanks anyway!

Crazy auctions sadly does not provide an API to allow me to easily block the backpack from being sold there.
It probably would be best if crazy auctions would be extended to allow more fine grained item blacklisting.