You could remind them that by law if they are not happy with their purchase, they can return the goods and are entitled to a full refund.
If they keep them, they have to pay the price agreed.
Put that in a letter, make it sound official and let them know that you are serious about it and will otherwise engage in legal proceedings. You may not have any other route but hopefully this step will be enough.
I am no expert in the matter: I don't know the nature of the goods and I am talking about consumer rights i am aware of btw!