Certain NPCs called Milmuyeoks appear in major cities during nighttime. These NPCs sell illegal trading contraban, which has a high value. This is the riskiest form of trading, as it flags you for open PvP and attack by guards for breaking the law. Illegal trading requires a higher trading level to participate in.
While participating in illegal trading, you will get an icon appearing over your head to signify your actions to other players. Be warned, you most likely won't want to come across any random people while traveling in this fashion.