Go to Hao Nam Street, there are lots of shops selling good qused pianos. I can personally recommend Huy Chuang Piano at Hao Nam Street 23. I recently bought a piano there and I am really happy with the deal and service so far (used pianos starting from 1000$, including chair, transport, first tuning and three year garantie). As a bonus, Mr. Huy Chuang speaks good English. If you're interested, you can also call the shop on 04 3911 0173 or 0912 178 187.