The official HDB Resale Portal is powerful, but most people use it wrong. Here is how to find the hidden gems.
Most people look at the general listing. But if you want a flat immediately, filter specifically for "Open Booking of Flats." These are unselected flats from previous BTO exercises that are available for immediate booking often cheaper than resale.
Sellers can list their flat for any price they want (e.g., $1 Million). Before you view, check the "Transacted Prices" tab for that specific block. If the last 3 units sold for $600k and they are asking $900k, you know there is room to negotiate.