There are always uncertainties when buying used parts from a motor breaker. This makes it all the more important to read and follow eBay's returns procedures correctly.
Did you open a case under eBay's money back guarantee? If so, you didn't need the seller to confirm receipt. They are a UK registered busiss seller so eBay would have required them to provide a prepaid, tracked returns label. If for some reason you preferred to return it yourself, if you added the tracking number you would still have been able to prove delivery.
If you didn't receive confirmation that seller was accepting your claim you should have escalated the case for eBay to step in. The fact that eBay closed the return window suggests that you didn't do this, and the case closed because of inactivity.