DPD

Hi, I hope you will be able to help me. I have ordered two nikon lenses from ebay seller (kachashop). My parcel was sent after very long time. The parcel allegedly was delivered to me Monday 15.01. Someone signed for it (x mark). There was no one in the household at that moment. The seller denies responsibility and don't want to make a claim to DPD because he claims it has been delivered. I have asked my neighbours and looked around the property but no one seen my parcel. DPD says they can not take my complaint as I am not the sender. What am I suppose to do in such a case.I will be very greatfull for your help. The parcel value was 719 pounds. Thank you Monika Wojcik