The tracking shows it was "delivered" to Heathrow. Hopefully, that actually means it was handed over to another courier who will relabel it and deliver to your address. But I've no way of knowing if that is the case.
You can keep asking the seller for more info on the tracking and for the Account Number, Contact Telephone Number, Email Address. (I expect you have already asked at least once. I don't see a problem in keep asking e.g. every 48 hours.)
If you don't get anywhere/nothing delivered in a few days, you can try talking to ebay customer services and tell/show them the parcel was delivered to Heathrow and not to the address you supplied.
You can also try contacting DHL possibly on https://www.dhl.de/en/geschaeftskunden/express/kontakt-express/online/sendungsstatus/sendungsstatus....
If you do, I would keep the question as short at possible/without any complaints - mainly asking if was handed over to another courier or will be delivered by DHL under another tracking number. If their answer doesn't help, ask where it was delivered, if they won't tell you - ask them was it delivered to <whatever postcode is on the ebay order>.
