non responsive seller

I bought a gas hob on 12th of Feb [ came with 2 year guarantee ] to fit in my kitchen when ready.

about a month ago we opened & fitted the hob only to realise it was illegal, there was no gas cut off sensor to sense a flame out. [ illegal under UK law ]

I informed seller & he stalled by saying he'd get in touch with his supplier. No response for ages so i threatened trading standards or Ebay, to inform them he was, under UK law selling illegal gas hobs !

We then got a reply saying sorry, send it back, for full refund.

Sent it back [ got proof ] & delivered 21st May. Have sent 2 messages asking them to refund me. No response !

They've had it a week now & i dont know what to do next ?