I wonder if any of the devices (assuming you still have them) might be a route to identifying where and when they were purchased.

The phone’s IMEI number for example, is unique to that specific device - so in theory there ought to be a way to find out exactly who sold it, where, when and ideally to whom.

Not that this is something for you to do yourself, but it could be a line of enquiry for police or (as I suggested in a previous comment) a lawyer.