The so-called open trading was none other than the students using their own abilities to exchange for credits.
An example would be the students of the Grand Desolate War Institution's Cultural Department, the Spirit Tattoo Artists who were proficient in Spirit Tattoo Technique.
When a student of the Combat Department wanted to engrave a spirit tattoo onto their body, they would go to the students of the Cultural Department and use their credits to buy services.
Likewise, when the students of the Cultural Department wished to acquire some kind of special demon beast blood to concoct "Tattoo Ink", they would post a mission and put forward "credits as rewards", and naturally, there would be many students from the Combat Department willing to go the depths of the Wasteland to hunt demon beasts.
