Under the official Monopoly rules, what happens when an unowned property is not purchased?

Under the official Monopoly rules, an unowned property that is not purchased is auctioned by the bank.

When a player lands on an unowned property, that player may buy it from the bank for the printed price. If the player declines, the banker must conduct an auction, and any player may bid, including the player who declined the original purchase. The property goes to the highest bidder, who pays the bank.

Many casual games omit auctions and simply leave the property unowned. That common house rule changes the pace and can make it easier for players to wait until a desirable property becomes available on a later turn.

An auction is not limited to the printed purchase price. Bidding can begin at any amount, and the winning bid may be lower or higher than the listed price. This rule applies to properties, railroads, and utilities when they are offered for purchase.