According to Zheng-Fu's knowledge, there were a total of three big monster soul shops. Two of the three depended upon the big clans. Those were Monster Heart Box Shop directly under the control of the Heaven-Burning Flame Clan, and Monster Jewel Shop directly under the control of the Celestial Star Sky Clan. As for Heavenly Soul Shop, it was not directly connected to any clan. They had their own forces to procure monster souls.
The shop that Zheng-Fu chose was the independent Heavenly Soul Shop. Because the Monster Heart Box Shop belonged to the Heaven-Burning Flame Clan that he had an issue with. If he went there and met the young man who had an issue with him before, something bad would definitely happen. As for Celestial Star Sky Clan's Monster Jewel Shop, Zheng-Fu had no issue with them. But it was under a big clan anyway.