Paul is correct, it matters not what the seller is doing with Royal Mail as per claims. His responsibilty is to you direcvtly and Royal Mail has no business with you specifically.
the seller should be refunding your payment immediately once agreeing that there is a problem.
some sellers forget this basic tenet: we can't kick the can down the road. We are responsible and no third party has any business in between our business.
Of course, that's why people like myself, Bruce, Sean, Gene Arnold, Heritage, WELN, Morrie Everett etc etc etc have such great reputations.
obviously, some people have a more casual approach to how they do business and so, they get neg feedback