After completing the process and officially posting the mission, Benjamin left the Mercenary Association.
The idea of putting up a "search for himself" was simple: when the unfamiliar youth appeared, he claimed to be a mercenary, and the "dame" seemed to be quite close to him as well. Perhaps this would be a good way to lure them out.
The Mercenary Association had a uniquely efficient way of dissipating information, a mission posted in one place was instantly receivable in another. Hence, even if the unfamiliar youth and the "dame" was in Regina, they too would be able to receive and accept this mission.
Benjamin didn’t expect them to jump right into his trap, but 200 gold coins isn’t any small sum either. Even if they’re not interested in the job, they’d at least "pop their heads out" for a peep by going to the Mercenary Association’s branch to enquire.
That act of "popping their heads out" is all Benjamin needs to catch them.