Is the intermediary who purchased goods from China – which have not reached the buyer after two years – responsible for returning the amount to the buyer?

As you acted as her agent, you did what you were commanded and did not defraud her, so there is no guarantee upon you, and you are not required to return the amount to her. She refused to resort to the Sharia court. If she supplicates against you, it is a sinful supplication, because a sinful supplication is not answered.