Umm, that was a special bag he could use to clone things. The only thing inside that was a knife he clones, a sword, and three other regulars, all three kind of don't matter to the plot so it's not really 'crucial' information. He sells it even so it doesn't matter.
Thank you...
I was just really curious coz I couldn't remember if it was revealed or not... and it was made so mysterious that I expected it was something important... (like when the first test when he had a problem bringing in the briefcase as he passed through a shinso wall) xD
Yeah, that was because of the three regulars whom he had hidden inside.
It's been so long since I started reading this and I kinda forgot something crucial...
Have we all seen what was inside Khun's briefcase in the first few chapters??? What was inside it again??? xD