If you haven't already made payment then the seller can get away with not replying to you, as it's against eBay policy to exchange contact details until you have actually paid for the item. If, on the other hand, you have paid already but the seller is being deliberately obstructive and won't provide you with the address that you need to collect the item from, then I would say it's more than likely that the seller is pretty cheesed off about the item only selling for less than half the amount that he actually wanted for it.
In view of the above set of circumstances I don't think it's likely that you're ever going to get the item, even if you have paid the seller for it, so I would agree with the advice that you have already been given with regards to opening an Item Not Received case and escalating it to eBay on day four if the seller fails to refund you in full by that point. At least that way you'll receive a total refund of the money that you paid to the seller.
