I don't care about the Mist World; I just want to be brothers with you forever

Profit is the initial motivator for all intelligent beings, whether human or ghost. Everyone comes and goes for gain. The promise of profits multiplied a hundredfold naturally gives rise to a frenetic cadre of ghost merchants. 

Due to the distance between ghost cities and the threats posed by hell warriors, such ghost merchants are scarce. Only those with unique talents who are unfazed by distance and threats from hell warriors can achieve this.

Typically, these ghost city merchants are backed by the city lord's mansion, because the venture is tied to their own interests. Unless the individual belongs to a major city lord, no one would easily break this unspoken rule. 

Hence, Alexander had Louis notify the ghost city merchants, knowing that only the intervention of the city lord's mansion could summon these traders from across the many ghost cities in Dark Hell.