An intermediary device for mediating a transaction of used heavy equipment may be provided. The intermediary device may provide a semitransparent photographing guideline including guidance on a photographing portion and a photographing angle for photographing a heavy equipment image on a heavy equipment portion basis to a sale terminal device. When the intermediary device receives heavy equipment sales request information including at least one of a nameplate image and a registration certificate image of the heavy equipment to be sold, an appearance image, first heavy equipment information, heavy equipment location information, and a desired sale price from the sale terminal device, the intermediary device may recognize at least one of the nameplate image and the registration certificate image to extract second heavy equipment information. The intermediary device may compare the first heavy equipment information with the second heavy equipment information to authenticate the first heavy equipment information, and calculate an estimated sale price in which a commission and a tax are added to the desired sale price when the first heavy equipment information is successfully authenticated.