CTLaw, you are assuming that in halacha we simply allow for שליח עושה שליח, in ALL circumstances, but that is not quite accurate.
Firstly, when one specifically appoints HIS rov, he expects his rov to be the selling agent.
Secondly, the agency is words and words cannot be transferred from one agent to another (therefore a shliach to make kiddushin cannot appoint another shaliach).
Regarding the “fraud”, the sale of chometz is not on SPRCIFIC chometz, but rather it is for “whatever chometz at location xyz belonging to abc” – if abc does not have any hometz,because it was already sold, then the goy isn’t buying anything. That isn’t a fraudulent transaction.