[thecatspajamas]

I've had it happen before, for reasons ranging from reasonable (previously sold and mistakenly re-listed) to unacceptable (I want to get more money than that). Never had one just cancel and not give an explanation though.

At a minimum, they have to file for a cancellation through eBay to get their fees back on the transaction. Ebay sends a request for your approval to cancel the transaction which you can either agree to (fees refunded to them) or not (they're stuck with the fees). Sticking them with the fees won't force them to go through with the sale, but may be some small consolation to you (depending on how expensive the item was).

Bottom line, if they refuse to follow through with the transaction, there is nothing eBay will do to FORCE them to do so.

Edited to add: I overlooked that you had said you had already paid for the item. In that case, go through the Item Not Received claim process as Steve suggested.

All of this is assuming that the seller still refuses to respond to you (he will have an opportunity to do so during the claim process). Sometimes mistakes or extenuating circumstances do arise that require transactions to be cancelled, but the seller at least owes you an explanation.