I have played Monopoly for years and never knew this rule.
The rules state:
BUYING PROPERTY… Whenever you land on an unowned property you may buy that property from the Bank at its printed price.
You receive the Title Deed card showing ownership; place it face up in front of you.
If you do not wish to buy the property, the Banker sells it at auction to the highest bidder.
The buyer pays the Bank the amount of the bid in cash and receives the Title Deed card for that property. Any player, including the one who declined the option to buy it at the printed price, may bid. Bidding may start at any price.
The benefit of this auction rule is that it speeds up the game a lot.
Instead of having to wait for someone else to land on the property, it immediately gets sold to the highest bidder.