The selling doesn't work.

I get this error: https://pastebin.com/3eQRwE7Z

As the error says, you were trying to acces it by the console.

But console doesnt have the same attributes as a player, do it throws an exception. That means that sum is wrong.
Either you created like something that runs like shop {playername} in the console when a player clicks sum or you were just trying to acces it in the console.
I hope that helps.

If that fixed it please close the issue, because we would like to keep the issue section clean.

This command must be sent from a player, and not your console. An extra sender check will be implemented in the next commit.