I have had a couple of buyers like that but have become wise to it now. I now say that I will give them 4 days grace to pay or the transaction will be cancelled and the item relisted. After 4 days you can open an unpaid case with ebay anyway, so there is no loss to you and if they really do want the item they will come up with the payment. If they don't answer your emails or pay up, I lodge an unpaid case with ebay. When the case is resolved I then relist them item and block the buyer.

I know it is inconvenient but keep your cool. Remember, there is no real loss to you, you still have the item and the satisfaction that they won't be bothering you again.