Sadly that happens. I'd guess you have been discussing this with seller.
EBay used to say "contact the seller by opening a case"
The instant you discover a fault you open a "not as described case". Seller sends a return label within 3 days. If he does not you escalate on day 4 and get a refund. If he sends the label you pack it up and send it off. The seller refunds within ?48 hours - if not you escalate the case.
There is no need for any contact with seller. He has - indeed - run you out of the 30 day time limit. EBay might say that it is up to buyers to abide with the invariable time limits.
All is not lost. If you used Paypal you can make a claim there up to 180 days and otherwise with your payment provider. @r_herrero