In some countries, such as Australia, auction is a common method for the sale of real estate. Used as an alternative to the private sale/treaty method, where a price is disclosed and offers can be made to purchase the property that price, auctions were traditionally used to sell property that, due to its unique characteristics, was difficult to determine a price for. The law does not require a vendor to disclose their reserve price prior to the auction. During the 1990's and 2000's, auction became the primary method for the sale of real estate in the two largest cities, Melbourne and Sydney. This was largely due to the fact that in a private sale the vendor has disclosed the price that they want, and potential purchasers would attempt to low-ball the price, whereas in an auction purchasers do not know what the vendor wants, and thus need to keep lifting the price until the reserve price is reached.