If you're actually in HK and can read Chinese, then Yahoo Auctions HK is an option. Usually you arrange to pick the stuff up at an MTR station and pay cash. Most things seem to be sold on a buy-now basis. Unlike Taobao, you're dealing with someone locally and don't have to worry about surprising extra charges on the "free" shipping.
As for used books, postage rate increases have made buying from the U.S. pretty expensive these days. U.K. international postage still seems reasonable and there are a lot of second-hand dealers in Britain with rock-bottom prices on common books. Plus, you get the benefit of the recent fall of the pound. On Abebooks you can limit your search to the U.K.