While discussing What happens if the auction winner cannot pay his/her bid? I concluded that the winning bidder cannot use the mortgage value of the property being auctioned to fund his bid.
Does the person landing on a property have the right to mortgage this property in order to buy it?
Buying Property Whenever a player lands on an unowned property, the player may buy that property from the Bank at its printed price. The player receives the Title Deed card showing ownership and places it face up in front of him/her. If he/she does not wish to buy the property, it is sold at auction by the Banker to the highest bidder. The buyer pays to 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 of buying it at the printed price, may bid. Bidding may start at any price.
The "face up" terminology seems to indicate that the property will not be in a mortgaged state at time of purchase.