Ok, long story short, just want to make sure my friend is taking the right steps...
She isn't English and speaks it badly so has asked me to help her with communcation. She won an item from ebay.uk on 17th December, and paid for it on 17th December. Seller advised her it had been posted 18th December by International Signed For as stated on insertion.
It hasn't arrived. She waited, (Christmas/bad weather in the UK/international delivery etc) and contacted her last week. Seller is in mad strop immediately and accuses my friend of scamming her "if she hadn't wanted the item she shouldn't have bid" and says it's all very suspicious that she has waited so long to contact her. She confirms item was sent "1st class and not second as it was too heavy for second" and reckons my friend paid only £4 for delievery whereas it really cost £8. Checking her paypal my friend actually paid £11.99 delivery (for signed for, not 1st or 2nd)
I have asked this seller 3 times now to confirm one way or the other how it was sent, because if it was sent signed for, obviously both this side and in the UK it can be traced. She doesn't seem to be getting that she is in the wrong by sending it ordinary post. Her latest message (yesterday) was that she has asked the PO if my friend's address exists 
and they are unable to confirm and is saying "if it was sent signed for it should have arrived" (which no-one is disputing, I'm sure, having had 7 ebay parcels not arrive here with normal post over the years, that there wouldn't have been a problem with signed for, the whole point is it was sent ordinary and is in the bowels of the Italian system somewhere, along with my 7 parcels)
Latest message this morning was quite abusive and said my friend is a scammer as her English has drastically improved (er, that would be because I'm writing the mails for her)
She should just go ahead and open a dispute now, shouldn't she? She's new to ebay and is scared she'll get bad feedback, even under a positive comment and people will believe the UK seller because she has 400 f/b and my friend only 6.