I'm not sure how used prices are in Melbourne but HK used gear are not deals at all. For example, a used 15mm Ltd was being sold at same prices new from bhphoto. The salesperson didn't even attempt to lower prices. Even old M-series were on the pricier end. About a year ago, I bought an M 50 1.4 for about $100USD that honestly, I would have valued at maybe $75 (I was a pretty naive young Pentaxian back then).
The cheap prices that you may see on ebay are only cheap because those stores do not have to pay for high rent. That being said, you may want to check out the local 2nd-hand traders forum
????? - ????????????????? - DCFever.com Not too many Pentax lenses but the prices will be very indicative of what is out there.
Also, in addition to what Arjay Bee has mentioned, be sure to check out "Sim City" Mall in Mongkok at Nathan Rd/Santung St. (the entrance is on Santung St.). That place is like photo camera heaven with a few used camera stores, including the one that offered the 15 Ltd. In contrast to the store Arjay Bee mentioned, this area is all about newer, all-digital gear. Great deals around if you're looking for stuff that isn't Pentax specific.
Enjoy your stay in Hong Kong!