I always list items for internatinal shipping because it increases the number of people bidding on the item. That increases the chance of the maxium bid being reached by UK bidders, which raises your final sale price.
I would say that international bidders win my auctions maybe 30% of the time as ultimately, the cost for interntional shipping lowers overseas bidders maximum bid and usually a UK bidder wins.
That said, I would still happily ship to HK. Use a fully trackable service like UPS, DHL or Fed Ex. You can’t get a fully trackable servce from the post office. International Signed For only tracks to the point it leaves the country and then gets you a signature for delivery. This isn’t a good enough solution for sending to HK.
Ultimately the cost for fully insured (note all courier companies will add this insurance as extra, likely for your amp this will be about £100) will be prohibitive but it will still increase bidding on the item and you will benefit.
Re.the suggestion of Pink Fish, you need to be a long term user to list items for sale on that site; if you’ve not posted more than 50 times you can’t list items for sale.
Ebay is the best bet really. I sold £13k worth of second hand kit via here last month and got 60% of the new value for it all (all over 6 years old).