It sounds as though you didn't read and follow eBay's returns procedures before sending them back.
eBay would have provided the return address - and if the item had been listed as UK stock, the seller would have been required to provide a UK return address. If it was imported direct from China you would have been responsible for the cost of returning it there.
When the postal tacking record confirmed delivery eBay would have issued your refund.
I'm afraid that it you try to return items yourself, not through eBay - especially to sellers in China - there are may things that can go wrong and you're on your own if they do.