have you checked their rating here to see if others have had a previous bad experience? the rating thread is pinned in the for sale/trade section
the policies PayPal has set up for users in your country will determine what you have the ability to do to remedy the situation. i'm assuming you have buyers protection, meaning you should be able to first make a claim in the 'resolution center' which alerts PayPal that there is an issue (and stops the clock from counting down to when you need to take this action), and also allows you to raise the issue to a dispute if you are unable to work things out through the Resolution Center. then PayPal will decide for you, which is pretty easy in this case because step 1 the seller will have to do is provide a package # they can reference and assure it has been shipped. if they haven't done this, they'd have nothing verifiable to provide to them. case closed, you'll get your money back. going through the resolution center is good too, because all the communication will then be done through the PayPal messaging system, and the case worker will review this during their decision-making process, and responses like that (and repeatedly missing deadlines) only strengthens your case
hope you can get it worked out though! life can get tough sometimes, consider that your seller might be having a rough go of it lately. happens a lot over the holiday time it seems. i'd keep trying until the day before you are allowed to still file a complaint in the resolution center, and then take that action and go through that process.