Story
According to Baison, Astin never intended to get involved in the power struggle between the Shugo and Aullaeu, and most of the trades took place in the Night Dew Cave. Go to the Night Dew Cave to gather information about Kochi's abduction.
After completion (spoilers)
"We found a document detailing an upcoming trade, so let's ambush them at the scene." According to Baison, Astin never intended to get involved in the power struggle between the Shugo and Aullaeu, and most of the trades took place in the Night Dew Cave. However, all you found in the cave were Shugo corpses, remnants of what appeared to be Aullaeu sorcery, and records of another unknown transaction. Astin was not merely trying to squeeze extra money out of Kochi, he had been conspiring with the Aullaeu on a much deeper scheme. You found a document in the cave suggesting a rendezvous between the Aullaeu and Astin, and an ambush was hastily planned for the appointed time.