For Sale By Negotiation
This is the most common sale method and also called 'sale by private treaty'.
If you are interested in a house that is for sale 'by negotiation' you can have a sale and purchase agreement drafted up and presented to the vendor at any time.
A House for sale by negotiation is commonly advertised with a price and an invite for 'offers over' (or similar terminology).
With 'by negotiation' there is no closing date for offers and no need for offers to be unconditional. Your offer will be submitted to the vendor and the vendor will consider it, taking into account the price, settlement date and conditions. They may then reject it, accept it, or countersign - and the negotiations begin.
In most situations you will be the only one 'in negotiations' with the vendor. If this is not the case, the real estate agent (or the vendor - if it is a private sale) should make this clear.
Often an agent will get you to sign a form acknowledging that you are aware you are in a competitive offer situation. If this is the case, you only have one chance, so you should put in your best offer.