Hi, I buy and sell on eBay and in this instance your first port of call should be to chase the Royal Mail, but please do keep your seller informed of what is going on. The package has obviously been in Royal Mail's hands as you had the card, so it's Royal Mail who have lost it. The comment above about leaving enough time to get back to the sorting office is fair enough so go back tomorrow, and Tuesday, and Wednesday, until they find it. IF it's gone missing completely then it has to be the person who sold it to you who raises the claim with Royal Mail (which is why you need to keep them informed), as the seller has to fill in a claim form for a lost package and provide details of the transaction cost, proof of postage etc. Personally I would not be happy selling an item of this value and only using second class post, I would want it by recorded delivery at least so it should be tracked, and it's good practice for sellers to ask the PO for a proof of postage so there is no doubt when it was mailed - and you also have the delivery card so keep hold of that, take a copy in case the sorting office asks to keep it.
I have had one instance where the Royal Mail mislaid a package addressed to me when I went to fetch it, it was missing - but I went back the next day and they had found it, so don't despair yet.
The other query is, did you pay by PayPal? as you may have some recourse there but I am not sure. If the claim has to go to Royal Mail to investigate then it can take some weeks to resolve and the money will be sent to the seller who then has to refund you for the value of the item. Postage is not included in the claim (or wasn't the last time I had to do this).
Sorry for the length fo this post but I have tried to give as much info as I can!