This is 100% correct, end of the day, the seller can choose any way he chooses to get the part to you. He could even pay a mate to come and drop it off etc. But at the end of the day unless you receive your item then you want your refund. There is no chance he can say you should be chasing Royal Mail etc as the sender has to chase them, not the would-be receiver.
Definately think twice before commenting on a public forum people. They can print off the comments (so even think about it when it's PM's) and these can be used as evidence. Also as a side note (not that anyone has) but be careful hinting at doing anything, either to the person or their property. Even if you don't intend on doing anything (which most people won't even when they say they will) because if sommat happens you'll be prime suspect
EDIT: Also meant to say, a seller should always send it recorded at least anyway. Otherwise what's to stop the buyer (I know we should live in a world where people can trust each other but obviously we don't) saying they've never received it when they have (Just to clarify not saying that this is the case in this instance). If that did happen the seller would have no comeback because they can't prove it one way or the other
Thanks for the comment Ian. I completely agree with everything you have said.