I have known YL for a long time and he is one of my most trusted friends and players in the game, he's one of the old guard, the ones that have been playing this game for the longest, (other members of this old guard are Jerek, itblobboy, Gret and jamman) so I highly doubt that he would risk his reputation, gear and money on here to hack you. On the other hand, derrickchoi has friends that have been banned for duping and bug exploiting, the notable ones being takker147 and likoh16.
Regardless of their pasts I believe that robby will be able to unveil this matter by tracing the pet and the tickets that were used in the trade between YL and derrick and I also believe he would be able to track where derrick's money went so this will be resolved soon, well hopefully, once he sees this.