Hi Stanley,
If your seller is at least reasonably experienced, he/she should be able to help you through the process. I would recommend you contact the seller and explain (you can probably just cut and paste what you've posted here), and ask:
1) whether the seller has any restrictions on shipping to Hong Kong, and whether they would be willing/able to remove that restriction temporarily to allow you to purchase. Not every seller may be willing to do so. In that case, request a cancellation. Since your payment obviously hasn't gone through, it should be a simple matter for the seller to cancel the transaction. Make sure, in that event, that you accept the cancellation message when it comes through.
2) if the seller has no restriction to shipping to Hong Kong (or if the seller can temporarily remove his shipping exclusion, which can be done), then ask if they would try invoicing you through eBay or directly through Paypal, as Paypal suggested.
(You may have a little more of a problem getting this sorted out if your seller is fairly new to eBay, in which case, if they can't or don't know how to invoice through eBay or Paypal, you can simply ask to have the transaction cancelled and look for the same item elsewhere).
Best of luck with this!