. The answer can differ depending on how you interpret the rules, since it's not clearly defined within the rules that Charles Darrow wrote when he first published MONOPOLY. But I'll describe how chief judge Phil Orbanes ran auctions at the 2009 US & World Championships.
When a player lands on an unowned property and decides that he or she does not want to purchase it at face value (either because they can't afford it, don't want it, or think they might be able to get it at a lower price), then that property immediately goes up for auction.
The banker conducts the auction and puts the property up for bid starting at $1, regardless of the original face value of the property. Any player, including the one who landed on it and declined to purchase it originally, can bid on the property. Any player can name a price, and there is no sequential order to how bids are offered. The auction continues until the banker concludes the auction with the highest bidder.
Once the auction concludes, the final bid is binding, which means that if the player does not have money to pay for the property, it is possible for them to have just gone bankrupt to the bank in the process of buying the property at auction.
You cannot mortgage a property you just bought to pay for said property; you must already possess the capital needed OR you can attempt to make a trade to raise the capital needed.
Assuming you have enough capital on hand, it's a very smart decision to always stay in the bidding at least until the auction gets to 50% of the face value of the property. This is because once you own the property, you can then mortgage it and receive the mortgage value from the bank. As an example, if you can buy Boardwalk (face value of $400) at auction for $200, that property cost you nothing as you can mortgage it and receive $200 from the bank for it. If you can get a property for less than 50% of its face value, then you'd actually make money by mortgaging it
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