If the seller is no longer a registered user then either the seller has closed his/her account, or eBay have banned the seller from the site. Your best course of action would be to open an Item Not Received case in the eBay Resolution Centre, then once the case has been open for eight days escalate the matter to eBay on the ninth day and they should refund you out of their own pocket. If the seller still owes eBay money then they will chase him/her for any money owing in order to ensure that they get their money back, so you shouldn't have to put up with being left out of pocket. The eBay Money Back Guarantee has been put in place with the specific aim of preventing this from happening in the first place.
In the event that you've left it too long to open an eBay case then open an Item Not Received case via PayPal. You get one hundred and eighty days to open a PayPal case, as opposed to eBay's thirty days, so if you do have to open a PayPal case then make sure that you escalate the case after two days if the seller is abusive, awkward, obstructive or simply refuses to respond. However, you should make sure that you escalate the case to PayPal before the twenty day cut-off point, otherwise if you leave it any longer the case will automatically close and you won't be able to open another one.
