The customer is sometimes liable for a purchases. If a person go and buy a known stolen item, pay money to known criminal activity/terrorists, then they may end up being punished for it.

The relevant question is what knowledge the buyer had, which the Court of Appeal did consider. Dyson UK companies and personnel was aware of the crime being done by the supplier.

The general legal question is not if a customer can be held liable for purchases. They can. It is how much due diligence is expected before someone should be held liable.