Getting a refund when item returned but you don't have a tracking number

[n.mott]

I received the wrong book from a seller in the US. They accepted a return and gave me a Freepost address in the UK. I posted via Royal Mail and got a Proof of Posting receipt at the P:ost Office, but ebay's Returns system keeps sending me to a page to enter a tracking number, which I don't have. The seller says they'll refund the book when they 'process the return' but that was 3 weeks ago and nothing has happened. How do I get a refund via ebay's 30 day guarantee if I don't have a tracking number?  

[red_magpie]

I thought that all Rouyal Mail parcels services now have a tracking number?

If you check your proof of posting, it should have a long number (I think always starting with two letters). This will be the tracking number. You can check on RM's track and trace web site whether it's been delivered.