Since all the files on Graal Depot are ripped from the game files, and we all know when a custom is uploaded to Graal then Toonslab/Unixmad/whoevertheowneris owns that custom, then this website has to be heavily monitored to not upload graphics already in the game files (customs and base customs) otherwise that is illegal, right? Selling copyright products when you don't own them becomes illegal, right?

So if I am correct, if any custom is sold on this website that has already been uploaded to any server once already, then this website is doing illegal deals. How does the owner of this plan on monitoring every graphic in the files of every server?

This is legally flawed as there is no way to easily prevent copyright customs from being mass sold. I would not recommend ever using this to purchase or sell customs.