Unfortunately, it is seller's responsibility. Also most of the cases, only senders can claim against courier/royal mail not the receiver. So, again, it is seller's responsibility unless the pickup/delivery was arranged by the buyer.
According to the UK gov'sOnline and distance selling regulations, seller is responsible for the delivery. To be exact, this applies to who arranges the delivery. If the selling condition was you send the goods to the customer, it is your responsibility. If the selling condition was pickup only and the customer arranged its own courier, then, as soon as it is picked up by the courier arranged by the buyer, your part is done.
Most of the cases, sellers are providing the postage on almost all ebay sales. In this case, if anything happens after the delivery, it may be arguable, but the customers are still protected with very strong 30days return window of "customer can return - nomatterwhathappenswhatreasons". Sellers are always lose unfortunately. This is a cheat key to the online customers.
So, I also gave up arguing with bad ebayers.
I started doing below.
1. I either send it by trackable service (which narrows my profit though). For items over 30 GBP.
2. For items valued below 30GBP, I send it by normal post and if customer claims, just resend it if that seems genuine and the customer has accummulated ebay feedbacks. In this case, I spend more time on the packing of the replacement item because if the products arrive with bad condition, negative feedback is guaranteed. Not trackable service could save around 2 GBP each shipping. So, sending additional when the problem happens was not that bad. The issue does not happen 10-20 times a day. eBay sellers are very careful when they leave feedback as they also understand other seller's difficulties. But people with nothing to lose on eBay would leave negative feedbacks easily.
3. I send refund, if the customer says that the item did not arrive and the ebayers are kind of new ebayers.
Even if you re-send the item, they may leave negative feedback.Claim agains Royal Mail for lost post. But you will have to go through so painful process to prove how much you bought it for and where and when and sold how where to whom... blablabla driving you xxxxy. For smaller ones, I simply chose not to claim for my mental health.
YP the active and great seller.